Thursday afternoon, Helen Lee left school early to go home and check the mailbox. She found the letter from Brown University she had been waiting for, but the news was not good.

"At first, I didn't understand what it was saying," said Lee, a brown-haired 16-year-old senior at Walt Whitman High School in Bethesda. "The letter said, 'we regret, we are sorry.' It didn't say we reject you outright. Then it finally hit me. I felt numb. I was in disbelief. I didn't expect to be rejected."

After the initial shock wore off, reality set in. Lee started making other plans. Her family is going to the mountains for the holidays, but Lee has decided to stay home and work on her applications to Princeton, Yale and Williams. "I don't mind," said Lee with a nervous laugh.

Lee is not alone this Christmas season. For many high school seniors, vacation will be spent filling out college application forms, writing autobiographical essays and generally worrying about the rest of their lives.

For them the anxiety connected with getting into college is not confined to April, the traditional time for colleges to send out letters of acceptance. It extends through their senior year, a year that can be a rewarding culmination to high school but a time that school counselors say is filled with an inordinate amount of anxiety and stress.

These days the anxieties focus on getting the applications done. Many schools have a Jan. 1 deadline, others want them by Jan. 15 or Feb. 1. Then the waiting begins, and students find themselves in a state of suspended animation -- ready to leave the comfortable womb of high school but faced with the uncertainty of not knowing which colleges will accept them. It's a condition one school psychologist calls "senior syndrome."

Early admission programs at schools such as Brown can relieve some of that anxiety -- if the news is good. But most seniors will not receive word on whether they have been accepted to the college of their choice for another three months.

"For them, waiting is interminable," said Penny Finch, a psychologist for the Montgomery County school system who serves as a counselor at Whitman and several other schools. "To adults, months seem like nothing; to them, months seem like eons."

Finch said that every year several seniors at Whitman and other Montgomery County high schools have nervous breakdowns and must be hospitalized.

"They break down under the pressure," she said.

The anxiety that goes with getting into a good college is even more intense in counties like Montgomery and Fairfax, where the economic and educational level -- and the expectations -- are high.

Of 6,899 Montgomery County seniors surveyed in 1983, 67 percent said they planned to attend college or junior college and another 12 percent planned to attend college part time.

Two percent of the seniors said they planned to attend Ivy League schools, and 17 percent said they planned to go to highly competitive public or private colleges or universities.

In Fairfax County, 69 percent of last year's seniors said they planned to attend four-year colleges and universities and 9 percent said they planned to go on to two-year colleges or junior colleges.

For many students, mailing the application is the final step in a long process that can be as thought-out as a game of chess. It involves carefully assessing their chances at particular colleges -- even to the point of knowing how many classmates have applied -- and often extra preparation.

Zachary Kittrie, 17, a senior at Whitman and president of the student government, said he took two special courses to help improve his Scholastic Aptitude Test scores and then took the test four times, scoring 1,140 on his final try.

"Each time I was hoping I would break 1,200," said Kittrie, who has been admitted early to the University of Michigan and plans to apply to several other colleges.

"The kids in the classes with you like to compare themselves to you," he said. "Some of them have 1,300 and 1,400 on their SAT, and it's hard to keep that in perspective and say, 'All right, in the United States there aren't that many people who make 1,400.' "

Other students apply to more than a dozen schools, hoping to increase their chances of getting into the right one.

James Dick, a guidance counselor at McLean High School in Fairfax County, where about 80 percent of the students go to college, said he has known several seniors who have applied to 15 colleges.

Because colleges require application fees that range from $15 to $70 each, these students spend a considerable sum just applying.

"We had two sisters here last year whose parents spent between $600 and $700 in application fees alone," Dick said.

John Keating, head of the guidance department at Whitman, where more than 90 percent of the students go to college, said the pressure is especially intense for students who have applied for early admission. At many colleges, if they are rejected they do not get a second chance to apply and, like Lee, they must scramble to get their applications in to other schools.

"For the ones who get rejected it's a blow to their self-esteem," said Lee.

Dick said he had to tell a student last week that Harvard was deferring his application. "It was painful," he said. "I could see it in his eyes. He looked down and said he didn't want to talk just then."

Finch said status-conscious parents can add to the pressure that seniors are feeling about attending college.

"It's a cloak a parent puts on," she said. "They tell their friends 'my son is going to Harvard; my daughter is going to Princeton.' A lot of parents live through their kids . . . . What college they get into frequently means less to a kid than it does to the parent, and the kid gets ensnared in that and he wants to do it for the parent."

Lee, whose father has a doctorate in economics, said a lot of her motivation for wanting to go to an Ivy League school comes from her parents.

"Of course they don't push me," she said, "but I just know they want it. I don't want to disappoint them."

Alice Gallin, 17, a Whitman senior, applied for early admission to Amherst but was deferred, which means that she will be reconsidered in the spring with the rest of the students who have applied.

Although she said she will apply to Penn State, Columbia, Cornell, Northwestern and Tufts, and she has been accepted by Michigan, she said she is disappointed that she did not get into Amherst on her first try.

Her father, a scientist at the National Institutes of Health, went to Amherst and had wanted her to apply there, Gallin said.

"He was disappointed I was deferred," she said. "He thinks I deserve to get in."

Like Lee, Gallin said she will forgo a trip to Florida with her parents so she can work on her applications.

Alvaro Puga, 17, a senior at Whitman who has applied to Syracuse, the University of Maryland, the University of Virginia and the University of North Carolina, said he strives hard to keep the competitiveness in perspective.

"Everybody feels the pressure, but you can choose to ignore it as much as you can," said Puga.

At the same time, he said, he is aware that "what I decide now will reflect on what I am doing in 10 years."

The pressure about college is off for Mike Klein, 17. He has been admitted early to Duke, his first and only choice. "I had to stick to a college on the Eastern Seaboard because my mom doesn't like to fly," he said.

But knowing where he will be studying next fall does not mean Klein can relax.

He said his acceptance letter warns him that it is contingent on keeping his grades up and not dropping any classes.

"I'll take it easy but not so easy that I slouch off," he said.