Unlike many students, and some teachers and administrators, Mason Nelson is happy to see the end of summer vacation and the start of school today in Montgomery County.

As director of the school system's supply and property management section, Nelson supervised the summertime transfer of 138,000 books, furniture and equipment, as well as countless files and instructional materials, from 19 schools closed by the board of education for the 1982-83 school year.

Just yesterday, workmen finished painting Rosemary Hills Elementary School, which the board had sought to close this year, while others in Nelson's section refurbished the county's last two-room school house, which despite a continued decline in enrollment countywide is being pressed into service because of growth upcounty.

The continued decline in enrollment, combined with school closures, has meant an unprecedented shift of 3,600 students and 1,400 school employes to new buildings.

"We are still trying to get out from under it," Nelson said yesterday, adding that he had to hire 40 summer workers to finish the task. "We had gathered 20,000 old boxes by June to move the stuff . . . that left us about 8,000 boxes short."

Enrollment projections in the nation's 20th largest school system show a decline of 3,419 students, to 92,168, compared to the system's all-time high of 126,311 a decade ago.

But because of changes in its attendance boundaries, Clarksburg Elementary faces a unique problem: too many students. So 40 Clarksburg sixth graders will be taught in the adjacent two-room, wooden structure that was built in 1909 and is the county's oldest school. It has been refurbished with its original, turn-of-the-century motif. "I think this is a great idea . . . something no other school in the county has," said principal Robert T. Stevens.

As the result of the unprecedented decision by the state board of education to overrule a closure by a county board, Rosemary Hills, long a symbol of voluntary integration efforts in the county, will remain open. Instead of the kindergarten-through-grade two pattern of recent years, it will house grades K-6, adding to it students previously enrolled in an ability-grouped magnet program at North Chevy Chase, which has been closed instead.

"We're ready," said Principal Richard O'Donnell, scoffing at suggestions that the hastily conceived arrangement won't work. "We have the creme-de-la-creme of both staff's and I don't see any problems."

The county board is appealing the state's reversal of the Rosemary Hills closure and boundary changes at Montgomery Blair High and Eastern Intermediate, but board president Eleanor Zappone said even if the county's wins in circuit court next month, there will be no attempt to move students before next fall.

A number of instructional changes will be in place for today's openings.

A revised high school English curriculum places more stress on writing skills; a new health education curriculum, minus a controversial plan to teach contraception to eighth graders, will teach cardiopulmonary resuscitation to all ninth grade students, and 10 of the 22 high schools will move from six to seven-period days, which budget director Ken Hill said will give students greater flexibility in course selection.

Hill said average class size in elementary schools will be reduced from 26 to 25 students because of lower enrollment and the addition of 18 new teacher positions.

Students will find a greater emphasis on computer related instruction, in an effort to match the growing demands for high technology jobs.

"Last year 45 of the 105 elementary schools did not have access to a computer," said program coordinator Beverly Sangston. "Now we will have a computer for every 200 students and give them 4.5 hours of instruction." The system will also test a plan to use microcomputers to manage the kindergarten through eighth grade math programs.

Because the number of foreign students has nearly doubled in the last four years, a fourth intensive English language center has been installed at Richard Montgomery High in Rockville.

Also implemented this fall will be the school system's new kindergarten to eighth grade policy, which has been criticized by some as a set of regulations that will stifle the creativity of teachers. A spokesman for program development and instruction says the policy is really a clarification of unspoken guidelines. Part of the policy requires homework assignments three to five times a week. Another stresses the need for pre-reading and math instruction in kindergarten.