When students at T.C. Williams High School, Alexandria's only public high school, return for the fall semester next year they will not only be barred from leaving campus most of the day but will find it harder to skip classes and still receive credit.

The Alexandria School Board, in apparent response to complaints about increasing truancy, approved a new attendance policy last week which no longer differentiates between excused and unexcused absences. Under the new policy, if a student misses 10 classes a semester he or she will automatically fail the course unless a special appeal is granted by the school superintendent. Previously, students were allowed an unlimited number of excused absences.

In addition to the tougher class attendance policy, the school board also unanimously voted to end the school's open campus policy. The school's 2,500 students, who previously were free to leave the King Street campus during any period when they did not have a class scheduled, will be allowed to leave only during lunch time.

The effects of the two changes, school board members heartily agreed, will be to insure that students are not only staying on campus, but also in class.

"It is the school board's belief that you cannot teach children when they are not in school, whatever the reason may be," said board member Claudia Waller before the vote that ended nearly a year of wrangling over the school's attendance policies. The new policies, Waller said, would make it more difficult to cut classes and encourage both students and parents to carefully consider any absence from a class.

Under the old attendance policy, students were allowed five nonexcused absences a quarter -- the system changes to semesters next year -- but could claim an unlimited number of excused absences without affecting course credit if a note signed by a parent explained why the class was missed. Included among the list of acceptable excuses were family vacations, car breakdowns, religious holidays and other nonschool activities.

The new policy no longer distinguishes between illness and nonsickness related absences, however. If a student took a family vacation and accumulated a number of absences due to sickness which -- when added together -- exceeded 10 days, the student would not qualify for credit unless the superintendent approved it. The same rule applies to religious holidays. Only serius illnesses or an injury, school board members stressed in the appeal policy, will be considered legitimate reasons for missing classes.

"People who have a number of religious obligations or want to make a family educational trip will have to make a greater effort to stay in school," Waller said.

The new closed-campus policy, other board members noted during the debate, also will encourage students to stay in class. The old policy, begun in 1971, allowed students to leave campus during unscheduled blocks of time and frequently resulted, critics complained, in students not returning for scheduled classes. Under the new policy, students will take more courses and have no free periods between classes. With the exception of lunch time, students will not be allowed to leave campus.

While the new attendance policies were heralded as a welcome relief to what is regarded as a troublesome truancy problem, some school administrators and school board members thought the 10-absence policy was still too lenient.

Efforts to decrease the number to first six and then eight allowable , absences failed only after an extensive debate which concluded with most board members agreeing that they would like to reduce the number, but that it was unrealistic to do so.

"Six may be ideal," said board member James Skidmore, "but 10 is realistic."