Tim Cooney was troubled. The 17-year-old boy from Potomac had just spent four weeks working in an orphanage in Mexico, cleaning mucus off the walls and eating tongues of beef, and he was afraid he had changed too much.

"I am so scared, man," he wrote in his diary, as he and seven other students from Gonzaga College High School, a private Jesuit-run boys school in Washington's inner city, were one day's ride away from home. "Never before have I been this scared as far as seeing my family and friends. I am just afraid of things not working out . . . . Man, I am so scared. I am so worried."

As he sat on a bed in Wheeling, W.Va., Cooney, the son of a neurosurgeon, whose family home has a tennis court and swimming pool, finally understood the meaning of a phrase he had heard often at Gonzaga: To be "a man for others." He had not thought much about poverty before, though there was a shelter for homeless men attached to Gonzaga and the children in Gonzaga's neighborhood were poor.

Now, Cooney, a tall youth with boxspring-tight brown curls, realized he had a responsibility to serve others. But something nagged at him. Friendships in adolescence were tenuous, based on a standard of sameness. Would his friends think he was a do-gooder? Would they stop accepting him?

Cooney had been thrilled that cold November day when Ted Skowron, the junior counselor and one of the chaperones for the trip, called to say he had been selected to go to Mexico. The Mexico trip had become a kind of special pilgrimage at Gonzaga where a student had to work in one of five neighborhood poverty centers to graduate or spend four weeks working at the Ciudad De Los Ninos orphanage. Some of the most popular teachers had been chaperones during the program's eight years and three times as many students always applied as could be accepted.

But Cooney knew that the trip would not always be fun when Skowron, 29, told him the names of the seven other students going with him. He had applied with his two best friends, but neither of them had been selected.

The only student he really knew who was going was Pat Ryan, and Ryan was a new transfer. Another student who was going, Jim Lyons, was too straight, Cooney thought. Kevin Quirk scared him because he was so religious. Matt Seiler chewed tobacco and Fred Brunton was a wrestler and seemed weird. Mike Leupold, who wanted to go to the Naval Academy, seemed too sensitive. Chris Deegan, with a wry sense of humor, was the one guy Cooney figured he'd like. Participants Selected

The eight included sons of a psychiatrist, two doctors, a former police officer, schoolteachers and a lawyer. All were white and only one lived in the District of Columbia.

For the next several months, Cooney felt uncomfortable as the eight worked concession stands at basketball games to raise money for the trip. Cooney was talkative and popular among the cooler crowd at Gonzaga, but often, he and the others picked for the trip would barely mumble to each other as they passed along hot dogs and Cokes.

When the group met in May in Pennsylvania on the last weekend retreat before leaving, Cooney began to worry that the relationships might be too strained.

But Skowron, known as "Skow," and Gerry McGlone, a Jesuit priest-in-training called "Bones," told the group that a wide range of students was chosen on purpose, from the smart to the sociable. The Jesuits' founder, Ignatius of Loyola, was a soldier who believed in testing; one learns best through that which is difficult, he taught. The students' first test was their selection. McGlone and Skowron wanted the students to learn to make sacrifices for other people, particularly for those they didn't know or didn't like.

During the retreat, the eight students set five group goals, one of which was to grow as a family and respect individuality without exclusion. The other goals were: "To break our hearts through service to others; to discover the assets of home; to learn about our vision of God, and to become aware of world poverty and problems that exist outside of America."

"I don't want to become different, yet in a way I feel I might . . . , " Cooney wrote during the retreat in the diary he was required to keep. "I don't want to come back all 'mellow' and my family and friends think that I think I am better than them."

As the day of the trip drew closer, Cooney thought about a story Skowron had told the group. Donald (Doc) Ward, the Jesuit who started the Mexican program, had been lounging in a bar not far from the orphanage, Skowron said. With him were the other adult on the trip, a Gonzaga student and a Ciudad De Los Ninos orphan.

A child approached the group asking for money. Ward and the other adult said they had no money. The Gonzaga student followed suit. But when the beggar asked the orphan, the orphan pulled a couple of pesos out of his pants pocket. Cooney wondered how he would have reacted.

Cooney was ready for adventure when the two station wagons pulled out of Gonzaga last June 15, beginning the week-long drive to the orphanage in the city of Guadalajara. Skowron and McGlone had planned the overnight stops to reflect a change in income levels as they neared Mexico.

The first night would be spent at a North Carolina campsite, the second with a suburban family in Georgia, the third in Louisiana and the final stop before crossing the border would be in San Antonio, at a Mexican-American parish where everyone spoke Spanish and the boys would sleep on wooden tables.

As the boys traveled south, tapes that Cooney had recorded of his favorite songs would not work, and he missed his girlfriend. Sometimes, he realized, he could be a grouch.

"Now I know about where half the group's sensitive temper lines come in . . . , " Cooney wrote June 17, after he and Chris Deegan had infuriated Kevin Quirk by hanging out the car window a stuffed animal he had won. "I've been pretty cranky myself, the past two days . . . . "

But nothing prepared Cooney for the contrast at the border. In Saltillo, a city of 192,000 and their first stop in Mexico, shacks the size of trash dumpsters leaned against skyscrapers. Some of the people sagged with hunger.

Yet, everyone was kind. One store owner invited the boys to her home and other residents took them to a fiesta and treated them like family. They have nothing compared to us, Cooney thought, but their Catholic faith is stronger.

The extreme poverty of Saltillo made their first look at the orphanage two days later a little disappointing. Although the Gonzaga students and teachers were assigned to the same L-shaped room with mattresses on the floor, the buildings were relatively new. The boys wondered whether the orphans really needed them.

"Are you a Catholic?" one of the older orphans asked Cooney the morning after his first night at the orphanage. When Cooney nodded, the orphan hung a chain with a medal of the Virgin Mary around his neck. "Un regala para vos a gift for you ," the orphan said.

Cooney knew the orphan was leaving that afternoon for the summer. So why would he be willing to give one of his few treasures to a stranger he would not see again? Cooney had come to Mexico thinking, "I'm an American, I'm giving them my time." Already, he suspected, the Mexicans would give more to him.

The pleasure of the first day hit a snag a few days later when the Gonzaga students were scheduled to paint the walls of the dormitories. First, the walls had to be scraped. "They're boogers!" someone shouted, recognizing nasal mucus on the walls. The group began complaining to the orphans who were helping them. "Nice country you have here," someone said. It was the most disgusting work Cooney had ever done. All he wanted to do was sleep.

News of the complaints quickly reached McGlone and Skowron. You are being obnoxious, the teachers told the boys at an emergency meeting. This is where the orphans live and you should respect that. The students said they expected to work on something big and have more free time than their six-day-a-week schedule allowed. A Student's Doubts

The meeting worried Cooney. Maybe he was not up to sacrificing what it took to do the work. Maybe he was too hooked to material things.

During the next 10 days, Cooney felt good sometimes and really low at other times. Mail from home was slow in arriving. But he was making new friends. Matt Seiler had become one of his best friends. Lyons had turned out to be one of the funniest guys on the trip and the biggest Romeo. He planned a side trip to visit some pyramids with Fred Brunton and Pat Ryan.

Cooney thought he was beginning to adjust better to being away from home. But his Spanish still was not great and McGlone and Skowron told him the first week of July that he was slipping into English too much. He was not meeting the orphans on their turf.

McGlone, a chubby man with a burst of brown beard, told Cooney he was not spending enough time with the orphans. Cooney, McGlone thought, was the quintessential suburbanite, the kind of guy who was great at cocktail parties but froze up one-on-one. But Cooney had an innate goodness that McGlone admired.

Cooney initially was defensive. Some of the orphans were obnoxious and Cooney felt used at times when an orphan would ask him to go to town and Cooney would have to pay for everything.

But Cooney knew his teachers were right. The orphans were generous more often than not -- offering him the last piece of watermelon at dinner, for example, when they obviously wanted it.

"I will totally work now," Cooney wrote five days later. "I have decided to put my whole heart into it."

During the remaining two weeks the students painted fences and buildings, worked in the garden and in a small adjacent factory and helped clean up the orphanage. Cooney no longer was bothered by the slow pace of the Mexican workers or the fact that they had to paint huge walls with little paintbrushes. He was used to setting physical goals for his work. Now he realized that his goals were internal, that the only important thing was that he was serving the poor.

As the end of July neared, Cooney and the other Gonzaga students were asked by Skowron and McGlone if they wanted to go to Puerto Vallarta, the Mexican beach resort, on their way home, instead of returning to the shacks of Saltillo. The students wanted to return to Saltillo.

Once there, the group drove to a mountainside village where Paddy Quinn, an Irish priest, was scheduled to say mass. During mass, Cooney could not take his eyes off one woman. Her infant urinated on her, but her eyes stayed on the priest.

After mass, the students doled out four handfuls of beans to each person. When one woman, about 60, returned for seconds, Cooney told her she was only supposed to have one helping. But the woman gave him such a mournful look he poured her more.

Later that day, as group members were eating ham and peanut butter sandwiches, Quinn and the villagers appeared. Cooney was embarrassed. They had eaten more in 15 minutes than the villagers probably had in the whole day.

That night at their last meeting before leaving Mexico, the group railed at Skowron about Quinn's bringing the villagers.

You are Americans, Skowron replied. Quinn is teaching you a lesson. How do you go back to the United States and live your life knowing poverty exists?

A hundred thoughts were running through Cooney's mind at the end of the meeting. He had toyed with the idea of becoming a Jesuit when he first arrived because the Jesuits were trying to help these people. But now he realized he could not be a priest or a missionary. He wanted to have a family.

Still, he was ashamed at how little he had done. Things would change when he returned to Gonzaga. Feelings of Guilt

For several weeks after he returned home, Cooney felt guilty for living in a nice house on four wooded acres. It bothered him that his friends drank beer and yelled at people at Bethany Beach, where his family vacationed soon after his return. Finally, he wished he did not feel so sensitive.

At school, Cooney entered a confusing period. He received a C in a religion class and it infuriated him because he thought he was more religious now than ever. Academics had been his top priority before going to Mexico; now family, friends and social issues ranked higher.

He argued with friends over the homeless and also over capital punishment, which he now opposed, and he organized dances to raise money for the poor.

Cooney was relieved when November finally rolled around and the group met for one last weekend re-treat. He had missed their feeling of family. More importantly, only the others could understand what Mexico meant to him.

During the weekend, Cooney wondered whether he wanted to be a lawyer anymore. He wanted to do something that would combine his personal responsibilities with helping others and he thought about joining the Peace Corps after graduating from Skidmore College.

By Sunday, the group discussions had buoyed his spirits and helped put Mexico in perspective. Some of his thoughts about the trip were still not formed and he could only guess at how much he would change in the long run, but of one thing, he was certain. He was a better person.

"They were poor as we were rich yet . . . they welcomed us into their homes without thinking who we were . . . , " Cooney wrote at the end of the retreat. "Who would think a poor Mexican would teach me a value . . . that was not apparent to me?"