Bruce Peoples, a junior at T.C. Williams High School in Alexandria, is a little worried about a new computer his principal will switch on at 6 p.m. next Tuesday.

The computer, called a "phone robot" by its makers, triggers a tape machine that automatically calls parents when their sons or daughters miss class. Until now, parents were not notified of absences until a student missed six classes in one subject, and then they were informed by letter.

The days when students lined up against the wall in front of the attendance clerk's office and forged each others' excuse notes is long gone, says T.C. Williams associate principal Gilbert Mays. "The computer will do away with much of the paperwork and expense of mailing notes home."

Peoples, grinning with a mischievous smile, accepted the title last week of "The Person Who Skipped The Most Classes at This Lunch Table." This new computer, he says, will make it more difficult to skip out on that the Spanish class that he finds "so boring" and a "waste of time."

Pondering the thought of the new robot resting behind locked doors underneath the auditorium's balcony seats, the handsome, blond student suddenly brightened as though someone had flipped on his own electronic switch.

"I do my homework by the phone anyway," said Peoples. "I'll just answer the phone and hang up."

Administrators agree that the computer they are leasing for $600 a month is not foolproof. It cannot tell who answers the phone, and if nobody is home or the line busy, the best it can do is call back three times that night. Still, they say, after many years and methods of trying to combat truancy, this phone robot is the grandest of solutions.

The number of unexcused absences has always been an elusive figure, T.C. Williams Principal John Porter says. When dealing with 2,400 students, he says, it is almost impossible to figure out which students are missing each period for an excused reason, like a dentist appointment, and which students have slipped out during lunch time deciding they would rather take a ride over to Old Town than sit through algebra.

Even without solid figures, Porter says, there are definitely some "habitual truants" and many who "skip once in awhile."

One statistic Porter plans to monitor, when the computer begins its barrage of calls between 6 and 9:30 p.m., is the number of students who receive no credit for a course because they missed 10 classes. During a single quarter last year, 360 students received the "no credit" mark, which translates into the accumulative average as an "F." If this figure drops significantly, Porter says it is likely that Alexandria's junior high schools will also invest in phone robots.

Parents are delighted with the new system, according to T.C. Williams PTA president Ruth Woods. She believes the computer will "help close the gap of information between the school and the parents and free up teachers for other things."

English teacher Patrick Welsh welcomes the computer. He recalls at least one old excuse that the computer may make obsolete: "The eggs were stuck in the frying pan. I had to stay home to get them out."

Alexandria's interest in the phone robot was spurred by the success of Arlington's pilot program last year. A survey showed that after the machine began calling parents, the number of unexcused absences on a given day in the three Arlington high schools dropped from about 26 to 16.

This fall, along with T.C. Williams, Alexandria's only high school, every high school in Fairfax and several in Prince George's County will be equipped with the attendance robot. In the District, where attendance figures are the lowest in the area, a federal grant will supply 52 schools with computerized phone machines this year.

Alexandria's robot will be programmed soon to call foreign-born parents in their own language, relaying the message in Spanish, Lao, Vietnamese, Khmer or Farsi, Porter said.

Back at the lunch table, Peoples and his buddies are busy planning ways to keep Mom away from the phone at night.

Even better, one student suggested, would be a way to switch the current message "Good evening . . . Your son or daughter was absent from one or more classes today . . . " to "Good evening . . . Your son or daughter is doing excellent work. We called to commend . . . "