It was Saturday and the last thing the Montgomery County elementary teacher expected was a call from the school janitor. There are kids in your classroom, he said. You'd better get down here.

As the teacher drove toward school, visions of classroom vandalism swirled before him. But what the teacher found when he peeked around his classroom door was not the mayhem he had imagined. There, clustered quietly in the corner, was the class pick-up basketball team -- doing math.

A little unusual? Maybe, but there was something else unusual about this classroom: It had a computer. In Montgomery County and elsewhere in the Washington metropolitan area, teachers are discovering that where there's a computer there's a line of students waiting to use it.

"When I first brought the computer into the classroom I tried to start a sign-up sheet. Kids could come in before school, after school and during their free time," says Frank Chisley, a teacher of gifted and talented pupils at Connecticut Park Elementary in Silver Spring. "Kids were signing up weeks in advance. It got so bad, I finally threw the list away."

Today's students face a world in which some experts predict that by 1985, nearly 80 percent of all jobs will require at least some knowledge of computers. Montgomery County administrators, concerned about the growing importance of these skills, are gearing up for a major effort to put computer terminals into most classrooms. After more than a decade of experimenting, this year, they hope, will be the year the computer finally comes of age in Montgomery.

A major evaluation of a decade-long pilot project has just been completed. School administrators found the results so promising that they plan to ask the county school board for more than $3.5 million over the next six years to buy micro- computers -- self-contained types that don't need to be linked to a central computer -- for all of the county's schools, and to expand their use in the curriculum. Currently, the county's computer instruction budget runs around $300,000 a year, and most of the classroom instruction is related to math classes.

There are now terminals in all 22 high schools and in 39 of the 122 elementary schools. Another 70 elementary schools have micro-computers for use in their gifted-and-talented programs for one semester each school year. That, say school adminstrators, is not enough.

"They're invaluable," said Beverly Sangston, coordinator of computer instruction in the school system. "We're looking at computers as one of the greatest potentials for teaching that we've had in years."

Most seem to agree that the push for computers in Montgomery and elsewhere around the nation was sparked by the development of the micro-computer in 1975. Unlike its parent system, which needs telephone link-ups to terminals from a large central computer with costs that can run in the hundreds of thousands of dollars, the micro-computer is self-contained and can be purchased for as little as $600. School officials estimate that with the advent of the micro-computer, the cost of student terminal time has decreased from $8 to $10 an hour to between 12 and 70 cents an hour.

The race to acquire micro-computers in some schools has become so intense that several parent-teacher groups have sponsored fund-raising efforts to buy the equipment for their schools. Sangston estimates that of the 200 micro-computers in Montgomery County elementary schools, nearly half have been purchased by parents.

Although Montgomery County has perhaps the most advanced use of computers in the area, its instructional program is not unique. All around the metropolitan area, school administrators are trying to to play a quick game of catch-up with the computer.

In Prince George's, at least seven terminals can be found at each of the county's 19 high schools, and they are used in math, science, business and language courses. There is at least one computer terminal in each of the county's 37 middle and junior high schools and the two vocational schools have extensive computer job-training programs. At the elementary level in Prince George's, computer use is still in the planning stage, but school administrators say a budget request will be made this year for micro-computers. Eventually, says computer coordinator Daniel Chase, Prince George's school administrators would like to buy micro-computers for all classrooms.

In Fairfax County, where computers have been used since 1967 in data processing classes, school administrators hope to be using computers in all ninth grade classes by June 1983. In the District of Columbia, a popular data processing course meets at Ballou High School.

"There's no question about it. The computer is the instrument of the future," said James T. Guines, associate superintendent for instruction in the District. "We've just got to get with it."

In Montgomery, Sangston and other administrators agree with Guines and say the county program has only begun to "get with it." In elementary schools, the computer is used mainly as a record-keeping tool for math scores -- students drill and test on the terminal and the results are stored in the central computer. Because of the limited number of computer terminals in this program -- there are 160 on the elementary level -- student time is restricted to about 15 minutes each week. Only in gifted-and-talented programs, such as the one at Connecticut Park Elementary, are students taught to write programs for the micro-computer so that the terminal can perform certain functions to solve problems -- in the classroom jargon, the students are trained in "computer literacy."

On the secondary level, Montgomery County high school students enrolled in geometry and algebra II classes have a three-week introduction to computer programming. Another 700 students take computer-science courses, in which they learn to solve problems with the computer and program it. One recent afternoon, students at Kennedy High School in Silver Spring were using the computer to calculate how much Social Security tax they would have to pay in various tax brackets.

"As far as motivation, it the computer is the most wonderful thing invented," says high school teacher Joy Odum, who also leads some of the school system's teacher training programs.

Although there are is little evidence that computer use improves test scores in all areas, Montgomery County teachers say they have noticed a rise in math scores. With a constant record of pupil progress at their fingertips, teachers say they can pinpoint a student's problem and provide tutoring in that particular skill.

"Teachers were a little skeptical at first," says Twinbrook Elementary School assistant principal and math teacher John Burley. "But when they saw how much time they saved and how specific they could become, they came around pretty quickly."

Now, Burley says, he can't get the terminals into the classrooms quickly enough. The students know the day the computer is scheduled for their room and anxiously await its arrival.

Not everyone is as optimistic as Burley about the use of the computer in elementary classes.

"I strongly advocate using it at the senior high school level and for vocational work," says Conrad Seeboth, Prince George's superintendent for math instruction. "But unless the computer is being used for something other than drills, it seems that to spend a great deal of money on the elementary level for computers that merely duplicate material that is already available in textbooks just doesn't make sense.

"Textbooks cost $10 to $12; terminals can cost as much as a $1,000."

Although many Montgomery County administrators would not agree with Seeboth, they say the most exciting use of the computer takes place when students learn to use it to solve problems, not when it is used as a tool to drill students or to store administrative records.

Last year during a summer computer program, 21 elementary students sat at 21 micro-computers four hours a day for almost three weeks. One pupil prepared a program that transposed a piano keyboard to the computer. The terminal keys paralleled musical notes. Hooked up to a sound box, the computer could then be played like a musical instrument. The programmer was in seventh grade and had had no musical training.

One fourth grader designed and programmed a tennis game that displayed volleying between two players. When a point was scored, a stick figure jumped up and "YEA" appeared on the screen. The student had also linked the game to a computer sound box, so there was background music.

"What is really remarkable is that you can take a child that feels good about himself and who is motivated," says summer program director Frank Chisley, who also uses similar techniques in his gifted-and-talented classes during the full school year. "You can give that kid some skills and -- wow -- when you put him on the computer, you've got yourself one hell of a kid!"