As 14,5000 students go back to class next week, the biggest change in Arlington County public schools this year will be the debut of middle schools and the termination of intermediate schools.

In the past, entering sixth-graders would have remained in one of 19 Arlington elementary schools. But as of this fall, they will join former intermediate school students from seventh and eighth grades, to make up the student body of the new middle schools.

The five new middle schools will be Jefferson, Swanson, Williamsburg, Kenmore and H.B. Woodlawn.

The change from intermediate schools is more than semantics. The emphasis in the middle schools will be on cooperative teaching, with a so-called team-teaching approach: Teachers will have 20 minutes of time together each day to coordinate lesson plans and to consult about individual students. The same students will be grouped together during most of the school day. Middle schools will have a longer instructional day, with more time for electives such as foreign language and art classes.

Also making a debut in Arlington schools this year is the controversial family life education program, still the object of a lawsuit by parents who oppose the plan.

The new, state-madated sex-education program was put in place for elementary school students during the latter part of the last school year, but it will see its first year of full implementation for all grades this fall.

The curriculums, for students in kindergarten through 10th grade, will cover such subjects as contraception, human reproduction, human sexuality, sexually transmitted diseases and child abuse. There is no family life program for grades 11 and 12.

Parents who do not want their children to take family life education courses will have the option of pulling them from the program at the beginning of the school year.

Arlington schools also have several new principals, some of whom assumed their new duties during the last school year.

In the county's elementary schools, there are four new principals: Michael Grinder at Drew Model School; Katherine Panfil at Key School; Laurie Baker at Tuckahoe School, and Meg Tucillo at Abingdon School.

In Arlington's middle schools, Carlton Lampkins will take the helm at Kenmore School and Margaret McCourt-Dirner at Williamsburg School.

There will be no changes in principals at the county's three senior high schools.

The county's school administrators are hoping that a new lottery will do away with wrangling over the limited slots at Page Traditional Elementary School. In previous years, parents have camped outside the alternative school's doors for several days at a time, in the hopes of gaining admission for their children under the school's first-come, first-served admission policy.

This year, however, admission to Page School for the 1991-92 school year will be determined by a random drawing to be held in November.

At the end of two years, the new admission policy will come under review by the county School Board.

Students at the county's two other alternative schools, Drew Model Elementary and H-B-Woodlawn, which has grades six through 12, will continue to be admitted on a first-come, first-served basis.

Siblings of students already enrolled at Page are to be admitted automatically under the new policy. Minority candidates at all three alternative schools will continue to be given preference.

Seven Arlington elementary schools will kick off their "exemplary schools" programs this year. The individually tailored enrichment programs will target schools with a high ratio of students who are considered to be at risk.

The schools, singled out because of such factors as transience, family income and percentage of non-native English speakers, include Patrick Henry, Key, Barrett, Long Branch, Randolph, Glencarlyn and Barcroft.

Among the projects, Glencarlyn will implement an all-day kindergarten under the exemplary schools program. The Barcroft school will implement its interdisciplinary Da Vinci Project, focusing on the contributions of the Renaissance painter and scientist.