Elementary school children in Manassas will find new science labs and computer classrooms when they return on Tuesday.

This year, city schools begin a program to teach all sixth graders to use computers. Last year, sixth-grade teachers were trained to teach that skill, and during the summer each of the four elementary schools in the district of 4,160 students was equipped with a classroom containing 25 computers.

Along with its computer lab, each school has a science lab where fourth, fifth and sixth graders will learn about physical and life science. "For the first time there will be formal lab time for younger students," said John Robert Rose, associate superintendent for instruction in Manassas.

New to each elementary school will be guidance counselors to help with special problems. And this year the city also begins a coordinated substance abuse prevention program for kindergarten through 12th grade.

A transitional grade one program has been designed for children who have completed kindergarten but are not ready to master first grade skills. And for children who need help even before kindergarten, there will be a developmental preschool.

According to Rose, a priority for the school system this year will be to move ahead on the seven-year capital improvement program adopted last spring by the school board. Construction on a middle school to accommodate 1,500 students will begin next spring for a September 1990 opening. School officials also plan to renovate the existing middle school as well as an elementary school, and to add playing fields and a vocational wing to the city's high school, Rose said.