With only seven students enrolled this year, the Advanced Placement European History class at Falls Church High School was an endangered species.

Principal Harry F. Holsinger knew that, with such a meager class size, he could not afford to offer the course again. He was prepared to ax the elective if the numbers didn't improve.

However, because a seventh daily class period has been added to next year's schedule, 39 students were able to sign up for the course, enough not only to save it but also to start a second section.

"We're in really good shape," Holsinger said yesterday. "We've been carrying classes with nine, 10, 11 children and now we're seeing some of those numbers growing to where they're reasonable class sizes."

Such has been the experience of administrators at many of the 44 intermediate and high schools in Fairfax County as they prepare to extend the school day by 30 minutes and a class period in September. According to figures released yesterday, many of the 60,000 seventh- through 12th-graders in the system are flocking to classes that attracted relatively low numbers of students in the past.

In schedule requests made this spring, high school enrollments in sociology, psychology, mathematics analysis, earth science, business law, computer concepts, advanced physical education, gourmet foods, basic technical drawing, art and photography all jumped by more than 1,000 students. Courses in creative writing, speech, drama, world civilization, algebra, computer mathematics and chemistry all increased by at least 500.

In intermediate schools, enrollment in journalism, speech and drama, civics, life science, art, introduction to foreign language, teen living and industrial arts all saw increases of more than 1,000, while pre-algebra, general music and seventh-grade chorus went up by at least 500.

Moreover, school officials reported that enrollment in computer science classes doubled and that 6,000 more students want to take keyboarding next fall. Enrollment also is up in German, Japanese and Latin.

Superintendent Robert R. Spillane, in his annual state-of-the-schools address to the School Board, used the numbers to argue that the board was right to approve the $16 million seventh-period initiative in spite of strong community and teacher opposition. The board hoped the extra class would allow students to take more electives at a time when the state is increasing graduation requirements in core subjects. Critics said they feared such a change would load too much work on students and teachers.

"Was it worth it? You betcha," Spillane told the board during its annual retreat in Fredericksburg, Va. "Was it difficult? You know {it was}. Did you do what the people said? Probably not. But you listened to a greater need . . . and said, 'You're going to like it eventually. Try it, you'll like it.' "

Principals and guidance counselors said that, judging from the schedule requests, the seventh period will save some elective programs such as drama and music that had been faltering at small schools, and will provide a major boost for business and computer science courses.

"These are the courses the kids really need and they haven't had the chance to get heretofore," said June Jones, a guidance director at J.E.B. Stuart High School.

"The sense I'm getting from the schools is that many students are using that seventh period to take the fine and practical arts classes . . . and explore things they would never have had a chance" to before, said Douglas B. Eadie, coordinator of guidance services for seventh- through 12th-graders.