It is an educational tradition of sorts for Yorktown High School in Arlington County to suspend much of its usual classroom gravity in the waning days of school to give its graduating seniors the kind of send-off that tucks neatly into lifetime memories.

This year, of course, is no different.

"They've worked 12 years to be a senior in high school," said Marilyn Henkel, an English teacher and sponsor of Yorktown's Class of '83. "They feel they rule the roost, so why not have a little fun?"

With that kind of official backing, this week found few seniors with their hearts in their class work. There was prom night, and many took a whole day to prepare. There was the live rock-and-roll band in the school gym and there was a group of senior boys dancing to the electric beat--attired in summer dresses.

For the next four days, such goings-on will continue. Yorktown's 300 seniors will have special invitations to school-sponsored dinners, brunches, swim parties and award ceremonies during a nine-day-long bash called (the uncertain arithmetic aside) Senior Week. And many students say they plan to augment the school festivities with weekday parties by night and casual class attendance by day.

"This is our final week in high school," says Yorktown's Andrew Laughlin, 17. "Now we have a chance to be free before we go on to more serious things. It's relief in the growing-up process."

Kathryn Scott, a dimple-faced senior recently named by her classmates as one of the school's "biggest flirts," said her final days as a senior have been "wonderful."

"We've been waiting for this since we were freshmen," she says. "I love this school, but I'm glad to get out. I'm excited to graduate, but there is some apprehension about it, too."

Minutes later, she sat in the school's darkened auditorium and giggled with her classmates during a special screening of a Yorktown High film production.

The film, called "Landshark at Yorktown High School," was written, directed and filmed by senior Barry Wheatmann. The eight-minute color film was made especially for a Senior Week student-faculty appreciation dinner.

Shot entirely in the school, the movie features dozens of Yorktown students, faculty and administrators being menaced by a cardboard shark that lurks in the empty classrooms and darkened hallways to the THRUM-thrum, THRUM-thrum, THRUM-thrum of the "Jaws" theme music.

"It's exciting," Angela Bronson, 17, said of the week's activities. "It's an excuse to be silly."

Eric (Moon Pig) Piecha was standing barefooted on stage, one of the 30 or so seniors who put themselves on the auction block this week before an auditorium of students in a frenzy to purchase the power to make a senior do something bizarre.

Wearing New Wave sunglasses, a black T-shirt with a monkey pictured on it and swimming trunks that barely concealed his long cotton undershorts, he paraded across the stage. As a willing participant in the "Maid and Butler Auction," Piecha said he believed he would bring a better price if he looked a bit unusual, which he did.

He and three of his friends, who as a group go by the nickname the "Yardapes," dressed in similar attire. They still went cheap, $6 for all of them.

"All through high school we saw people doing this," said 17-year-old Eric Olmon, one of the group. "Now, it's our turn."

Laughlin, also a Yardape, said he wishes he could make these days last forever. He will be going to Randolph-Macon College in Ashland, Va., in the fall. "But it won't be the same."

One reason it won't be is because he'll have to leave behind two of his best friends, Olmon and Todd Hitt.

All three live in the affluent Country Club Hills neighborhood near Yorktown High School. All three promise they will never lose touch with one another despite the hundreds of miles that will separate them when they attend different colleges in the fall. All three say they believe these last days will be remembered as some of the best ever.

"I'm happy. I'm really happy," Hitt shouted on a recent school night spent in a Georgetown nightclub with his friends. "I'm as happy as I could be. I couldn't be having a better time.

"I think I am one of the luckiest kids in the world," he said. "It's Senior Week now and everybody is real cool about things; none of the teachers are putting anything real heavy on us. Most of the teachers are being real nice."

Said Hitt, "I feel like the rest of the school is not even here."