Students returning to school in Fairfax County next week will find more computer programs, fewer vocational education courses and expanded sex education classes.

School officials also plan more training programs to help teachers identify troubled students, in an attempt to curb alarming increases in student suicides. In county high schools, a push to ban student smoking areas in secondary schools will be part of an ongoing antismoking crusade.

Fewer new students will be entering the Washington area's largest school system, which expects about 122,000 students this year compared with 124,426 last year, leaving administrators with headaches over surplus teachers and possible school closings. Five schools will not open their doors this year because of declining enrollments, and officials say they face more school closing decisions this year. At the same time, four schools are being built to cope with rapidly developing areas of the county.

Several changes have been made in top administrative offices for the school system. The biggest change is in the superintendent's office, where William J. (Jack) Burkholder, a 26-year veteran of the school system, was selected to replace Linton Deck, who served a controversial 2 1/2-year tenure before resigning this summer.

For students, the most important changes for the 1982-83 school year will be in the classroom, many resulting from months of political debate between school officials and local groups.

After five years of controversy, sex education programs were liberalized and quietly introduced through pilot programs in nine high schools last year. The courses, part of regular biology classes, will be expanded to all 23 high schools this year.

Last spring's budget wars left the school system with fewer new computer programs than former superintendent Deck wanted, but with more than the system offered last year. Computer literacy courses will be introduced in intermediate schools, and experimental computer programs begun last year in several high schools will be available to all high school students by next spring, said Thomas McGarry, director for curriculum development in the school system. Some elementary schools will offer basic computer literacy courses.

While the school system is expanding some academic courses, it has cut a third of its vocational technical programs. Vocational courses being discontinued include business mathematics, tailoring, fashion design, small business ownership, furniture marketing, needle crafts, catering, introduction to horticulture and marriage and parenthood.

School officals said the programs were dropped because of lack of student interest, although many topics in the vocational sequence will be incorporated in other courses. Critics of the decision to eliminate 32 courses said the school system did a poor job of promoting the programs. As a result, schools have been ordered to upgrade their advertising of vo-tech programs.

On the nonacademic side, school officials have decided to launch tougher antidrug and antismoking programs. Antismoking forces have been gaining momentum in the schools: five high schools voted to abolish student smoking areas last year.

"There's a very strong push among parents and people in the community to do away with smoking areas in the schools," said Alvaine Hamilton, a spokeswoman for the school system.

The decision to ban smoking, which is made by students, parents and officials at each school, is creating new problems and costs for the school system. Schools with smoking bans must hire aides to patrol hallways and restrooms to prevent youngsters from sneaking cigarettes. Sixteen secondary schools now have designated smoking areas.

School officials also are attempting to provide more help for troubled students following several student suicides last year, including one incident involving a sixth-grade student.

"We want to alert teachers to the predictable signs," said Beatrice Cameron, assistant superintendent for student services and special education. "We will not be providing counseling, but will work with community resources in a preventive manner."

The school system will offer programs to help teachers recognize potential suicide victims as well as programs to help students and staffs and schools where such incidents occur.

In an unexpected move, the school board decided not to increase school lunch prices this year. The school lunch program ended last year with a slight surplus, despite declining student participation brought on by last year's price increases and decreasing federal funds. School officials credited the new a' la carte lunch with attracting many students who dropped out of the regular lunch program and helping keep the program in the black.