When Carlos Mendez came to the United States from El Salvador 18 months ago with his family, he spoke little English. His parents enrolled him in Woodrow Wilson High School in Northwest Washington, he had trouble keeping up with English-speaking students.

He attended a special English class for foreign students, one period each day, but that didn't seem to help. His teachers "spoke too quickly" for him, he recalled, and he began to think they disliked their Hispanic students because sometimes the teachers wouldn't stop to explain things twice when he didn't understand. He began to fear that students in the class thought he was "dumb."

So Carlos Mendez soon became part of the core group of Hispanic students at Wilson who -- frustrated by what they perceived as a lack of understanding -- would come to school each morning on time but would never go inside.

Instead, Carlos would drive up in his silver-blue, white-striped station wagon, park in front of the school, flip on the car radio, and listen to Michael Jackson singing "I Wanna Rock With You" on a FM disco station.

Sometimes he would take a break and drive over to Munchies, a lunch spot on Wisconsin Avenue that is a favorite hangout for Wilson students, for hot chocolate.Lunchtime was the only time that Carlos would go into the school building -- to but a 40-cent lunch and socialize.

To help students like Carlos, who is 17, before they drop out of school altogether, the District school system, together with the national Hispanic organization SER, is opening its first billingual school for high school students.

The school, which begins holding classes today, will eventually serve 150 minority students, most of whom will be Hispanic. About 50 students are enrolled in the school, which is housed at the Marie Reed Learning Center at 18th Street and Kalorama Road NW, in the heart of the city's Hispanic community.

Most classes will be taught in English, but teachers and students will have the option of using Spanish. There will also be a heavy emphasis on individualized instruction and instruction in foreign cultures, particularly Hispanic culture.

"I come [to the bilingual school] because I think I get more help here," said Julio Nunez, a 10th grader who came to the United States a year ago from El Savador and was failing at Wilson.

The District has a bilingual elementary school -- the Oyster School in Northwest -- which is now about equally divided between American-born and Hispanic students.

But over the last six years, some Hispanic leaders have called for a similar program at the high school level for the growing number of teen-agers coming directly into the public school system from Latin American countries.

This teen-age group poses a special problem for the schools, since some of the youths, because of their age, are placed in junior or senior high schools here, even though they may have only received up to a fifth-grade education in their own country.

The development of a bilingual high school is only one indication of the tremendous changes occuring in the Washington area over the last decade. Schools systems in metropolitan Washington must now cope with ever-larger numbers of children for whom English is a second language. Five percent of the total public school enrollment in the area -- 21,628 pupils -- are citizens of foreign countries.

About one-third of the students in the new bilingual school are transferring from Wilson High School, which has the largest percentage of Hispanic students of any school in the city.

Wilson has 158 Hispanic students. Each year since 1974, the Hispanic population in the school has fast approached the enrollment of white American students, who number 349 now. Since 1971, the majority of Wilson's student body had been black and there are 1,133 blacks in the school now.

As a result, Wilson, which is considered academically sound, faces a number of delicate problems and unspoken tensions among its student body.

Hispanic students complain that teachers will not give them extra help in class or after school.

"The teachers are always in a hurry when I want to talk to them," said one Hispanic student at Wilson who is still struggling with English. "They say they have to go to lunch or go home."

But the teachers maintain they would be happy to stay after school and help their Hispanic students if only some would ask them. The teachers said they make a point of calling the student's home when the student has not been showing up regularly for class.

And two teachers said they had taken special courses recently to help them deal better with the increasing number of foreign students in their classes.

American-born students at Wilson, who the Hispanics say won't mix with them, maintain it is the Hispanics who stick to their own cliques, where they can speak Spanish.

"It sees like they're lost. They won't relate to anyone else. They segregate themselves. They talk in Spanish to Spanish kids," said senior John Ropse, who is black.

The fact that there is little communication between Hispanic students, their teachers and their peers does little to change the perception each group has of the other.

The Hispanics have a cluster of tables in the cafeteria where they sit among themselves, away from the blacks. They also gather at one special tree in front of the school before school begins and when it's warm enough to go outside.

This voluntary segregation -- which also exists to some extent between black and white students at Wilson -- does not cause Wilson officials to worry.

"We are a school of minorities," said principal Maurice Jackson. ". . . I don't think you can get whole groups to mix."

It is just this atmosphere that many Hispanic students say they hope to escape by attending the new bilingual school, which is being funded with a $100,000 annual grant from the U.S. Department of Labor.

Teachers say their experiences can be just as frustrating for them as they are for the Hispanic students who are struggling in their classes.

In one English class, for example, Marion Hatcher, the teacher, took her foreign students aside recently and said she wanted them to write a composition about what school is like in their native countries, and how students there spend their leisure time.

In the process, Hatcher twice explained the meanining of "homogeneous" and spelled the word for her students. When she asked one of the boys in the class from El Salvador to repeat the meaning of homogeneous, he looked at her blankly and said, "I don't know."

Hatcher then explained the word a third time and had the boy repeat the word and its meaning twice.

Some of her Hispanic students have complained that Hatcher only calls on the English-speaking students in class. But Hatcher maintains she is torn between trying to encourage the Hispanic students to speak up in class and not wanting to embarrass those with a language problem in front of their peers.

"Sometimes I'll call on a student and he'll just sit there and stare at me for five minutes. In the meantime, all eyes in the class are turned on him.So sometimes I'll just call on someone else, just to let the kid off the hook," Hatcher said.

Other teachers have apparently exhibited less concern for their foreign students, according to many of the Hispanics. Erick Alvarado, a senior fluent in English, recalled one incident in which a Latin American student asked his teacher a question and the teacher curtly responded, "I repeated that before. I don't need to explain it again."

"Some teachers have very little patience with foreign students," said Lois Carroll, a teacher of Spanish at Wilson. "I've heard teachers say, if they don't know English, fail them."

Still, school officials -- and even the students -- are divided over whether creating a separate high school for foreign students is the best solution.

One Wilson student, Silvia Lopez, had decided to attend the bilingual school because she was having difficulty in some classes and was in danger of failing. But after enrolling in the new school, she decided at the last minute to stay at Wilson, because, she said, "I want to learn English very, very well and I fear if I go (to Marie Reed) I will speak more in my Spanish."

Many teachers have said they believe it is better to put foreign students in an intensive English class when they first come into the school system so that they can speak and understand English sufficiently before they are mixed in a classroom with English-speaking students.

According to its charter, the bilingual school at Marie Reed must be used for students with more than just language difficulties. It is aimed especially at the 16-to-21 age group, who can no longer be forced to go to high school. And, a large part of the school program will be aimed at gearing these youths toward a career.

School officials have had difficulty getting many of their Hispanic students over 16 to choose going to school over getting a job, even when it is not necessary for them to work to help support their families.

"Many come from deprived countries and didn't dream of being able to have a car or other things that they have here that would be considered luxuries in their country," said Wilson's only bilingual counselor, John Queralt.

"They are confused a little bit by the possibilities they have in this country. They try immediately to possess things they think can give them status."