There will be fewer students returning to Prince George's County schools next Monday--the count is expected to be down by 3,000, to 108,000--but school officials say more will be demanded of them. Elementary school pupils will be spending 15 minutes more a day on math instruction, for instance, and teachers at all grade levels have been directed by Superintendent Edward J. Feeney to introduce and reinforce math concepts as they teach other subjects.

All students, starting with this year's ninth graders, will be required to take a third year of math and fourth year of social studies in high school as a condition of graduation.

Last winter, the County Board of Education tightened graduation requirements, increasing the number of required courses from 12 to 14. In addition, the board passed a rule forbidding a students to enroll in a course below his level of ability.

"In the past, they were able to slip by and take courses below them," school spokesman Brian J. Porter said. " . . . We had a tough time ordering them out of such a class."

School officials insist much of the tightening of the acedemic programs have been in the planning for years but note the new emphasis on math and science has come largely in the wake of recent national criticism of public education in general and Prince George's poor performance on last year's Maryland Functional Math test.

Nearly 75 percent of county ninth graders failed that test, which was given on a pilot basis last fall. Passing the test, which will be given to ninth graders again this fall, is now a state requirement for graduation beginning with the class of 1987.

Teachers will be asked to do more this year as well. A series of special in-service training classes will be given to math instructors with the specific aim of improving student performance on the functional math test. A new teaching position, that of "teacher coordinator," has been created in high schools this year to help teachers of math, science, social studies and English develop and improve their teaching skills under the guidance of a senior teacher in that subject.

The senior teachers, who will receive special incentive pay, will teach half time and spend the remainder of their day working to improve their departments. The senior positions will replace the functions of department heads, who previously had full-time teaching responsibilities.

The teacher coordinators' tasks will be aided by 97 more teachers provided for in this year's budget. However, class sizes, which grew sharply after 500 teachers were laid off in 1982, may not be improved in most schools, school officials said. Additional money also has been found for textbooks (no new ones were bought last year) and other critically short school supplies.

"It's not as much as the Board of Education had requested, but more than we had the year before," Porter said.

The continuing budget problems will be most noticed in the maintenance of the schools, Porter said.

"They'll be as clean as we can keep them, but there won't necessarily be budding flowers on every lawn or new paint on every wall," he said.

Other signs of the budget crunch will be a new, one-time $5 activity fee for secondary students who participate in extracurricular activities and the elimination of more than 50 administrative positions this year, for an estimated savings of $2 million.

The system also will continue to shrink as enrollment declines. Eleven schools closed last June will not reopen this year, bringing the number of buildings in use by the school system down from 186 to 175. Two of the school populations, those of Landover Hills Elementary and West Lanham Hills Elementary, will attend classes at the former Glenridge Junior High School.

"We lowered the doorknobs and everything," Porter said.

Finally, there will be only one junior high school, Surratsville, left in Prince George's this year. The remaining 28 former junior highs, which used to have grades seven through nine, have been converted to middle schools with grades seven and eight only, completing a three-year planned changeover.