Cristal Mendlen sometimes wears sweat pants from her former high school with a sweat shirt bearing the emblem of her current school to show that she got the best of both worlds when she was forced to transfer from Jefferson to Annandale for her senior year in Fairfax County.

But Melodye Phillippe still remembers how she got lost in the hallways during the first few weeks of class last fall, after she and Jefferson's 180 other seniors moved en masse to the school that had been their traditional sports rival.

"It's like we're freshmen again," she said.

The transfer of so many high school students so close to their graduation was an unusual one. At midyear, most students appear to have made the adjustment -- some happily -- after three years of familiar friends, classrooms and athletic fields.

However, many former Jefferson students say, with varying degrees of resentment or resignation, that they were cheated out of one of the prized properties of adolescence: a real senior year.

The 12th grade of high school is traditionally a season to preside over younger students and to bond for the final time before leaving for different destinations and responsibilities.

Instead, some students said, they felt like strangers to most of the teachers and teen-agers at Annandale when they moved from an underpopulated school of 600 students to a building with 2,300.

"The trouble I had was not knowing my teachers," Phillippe said.

"That shouldn't happen to a senior . . . . In Jefferson, I knew everybody at least by name. At Annandale, it's the opposite. I was very depressed the first few weeks of school, not knowing anybody."

Like other students, Phillippe does not like the crowding that prompted school officials to install six classroom trailers, add a third lunch period and tack a minute onto the passing time between classes.

Although she was elected senior class treasurer at Annandale and said she has adjusted to the change, she said she misses the closeness at her old school.

"What are you going to do if you fail a test? Who do you run to if you can't find your close friends?" she said. Phillippe wonders whether any younger students will want to visit her at college.

The scenario of school mergers and boundary changes is an increasingly familiar one in the suburbs, as older neighborhoods lose their children and newer subdivisions bloom with baby strollers. This is especially evident in fast-growing Fairfax County, where two high schools have closed since 1985 and a new one will be among the seven new school buildings scheduled to open next fall.

The reason behind school boundary changes may be logical -- in this case, two schools with declining enrollments were merged into one -- but the human impact does not always follow the rules of reason.

"At first, it was really hard on everybody," said Laura Hutchison, an Annandale senior who is coeditor of the school newspaper. But she said it has worked out: "There are so many neat people I wouldn't have met otherwise. It's making our school better."

School mergers can be difficult for teen-agers because so much of their identity is linked with their school and they have not learned to cope with change as well as adults, said Dr. Ralph E. Wittenberg, a District psychiatrist and a clinical professor of psychiatry at George Washington University.

"It's easy for adults to look at kids and say, 'How trivial this is,' without remembering what it was like," Wittenberg said. "We don't like to think back to how easily upset we were. This is a big deal for a kid."

The sequence of events at Annandale-Jefferson began in 1984, when the School Board voted to convert Jefferson into a competitive-admission state high school for science and technology and send its students to Annandale. Jefferson was phased out as a regular high school over three years, under a plan drafted with PTA approval.

Beginning with the class of 1989, students were sent directly to Annandale. Members of the class of 1988 were caught in the middle: They spent three years at one school, then a senior year at a new school. Fifty Jefferson students were so reluctant to leave that they took advantage of a program offering seniors a one-year, intensive curriculum at the high-technology school.

Compounding the wedding jitters were the suicides last fall of three senior boys, two from Jefferson. Mental health experts emphasized that there is no evidence that the deaths were related to the merger, but the fatalities were a shock to the school just as it was struggling to settle into normality.

After the three suicides, Annandale officials tried to relieve the grief with assemblies for each class, small-group student meetings with counselors, and an information session for parents. At the student assemblies, some teen-agers began to cry and embraced classmates they had never met.

In the end, students said, the bereavements drew the school closer together. "It's unfortunate that that's the reason we've become so close," said senior Julie Thompson.

Raymond Watson, Annandale's principal, said the school will make additional efforts during the second semester to help students get acquainted because "some kids are saying there's a continued need for that." He said teachers will be asked to "make sure the kids really do know each other."

Extraordinary steps already have been taken to acquaint students, a task that was eased by the fact that Jefferson and Annandale are only a few miles apart in central Fairfax County, inside the Capital Beltway and west of Shirley Highway. They had demographically similar populations of mainly white, middle-class students. Jefferson students were brought over for a day last spring to tour their new school. Some electives that had been offered at Jefferson were added to the Annandale schedule, such as a full-year psychology course.

Jefferson students will receive Jefferson diplomas and be ranked with their old classmates, and they were offered the chance to purchase class rings with the seals of both schools. Two of the four senior class offices were set aside for Jefferson students.

One result of the effort was that the student newspaper recently gave Watson an A for his handling of a merger that "could have been very difficult for everyone."

Some seniors from Jefferson say they are better off at their new school. Mendlen, for example, is senior class vice president and sports editor of the Annandale school newspaper. "The majority of my friends are {from} Annandale," she said.

Senior Kelly Dimock said she also is glad she went to Annandale: "I've made some really good friends." Dimock did have one disappointment: She played varsity basketball for two years at Jefferson but did not try out for the Annandale team because "they had a state team and I didn't want to sit on the bench." She said she plans to try out for the soccer team instead.

Some other Jefferson seniors also said they were squeezed out of sports because of the more competitive atmosphere at Annandale, a traditional Virginia football power. School athletics director Bob Hardage said Annandale seniors dominate the sports teams by numbers (Jefferson students make up less than a third of the 750 seniors at Annandale), but he said Jefferson students have done well. Of the seven seniors from Jefferson who had been on their former school's football team, four "participated considerably, and one was our backup quarterback" at Annandale, he said.

Jefferson's recent football seasons were not standouts, in part because the school's enrollment had dwindled; by contrast, the Annandale Atoms made it to the district championship game last fall. "Those {Jefferson} kids were very appreciative of being in a situation where they had a chance to win," Hardage said.

Counselors, teachers and other officials said that the merger was bound to cause some problems, but they said the students have weathered them.

"It's a loss like other losses, and you have to recognize that kids are affected by losses," said Myra R. Herbert, social work services coordinator for the county school system.

Privately, some school officials say they will think twice before approving a merger that sends students from one school to another for their senior year in high school. Some students say they are glad about that hesitancy. "I was mad that they didn't bring you guys over junior year," senior class president Sam Uzabel told Phillippe and Mendlen.

"As positive as this has been -- and this has been positive -- I question the validity of the merger," said Nancy Grim, who taught at Jefferson for 13 years before going to Annandale. "I've seen kids stretched by this in terms of personal growth. I just wonder whether it has been worth it. I don't know the answer . . . . I just feel if you can avoid it, avoid it."