THE PRINCE GEORGE'S County public school system, which has one of Maryland's lowest per-pupil expenditure rates, has released figures that show that its third-, fifth-and eighth-grade classes surpassed the national averages on every subject area of the California Achievement tests. Since 1980, the Prince George's students have shown steady improvement. Current third graders, for example, scored 10 percentage points higher on the reading test than their counterparts in 1980. The school system's ninth graders continued the success of last year's class with a passing rate of nearly 91 percent on a state-required functional reading test.

These improvements, after 39 percent of the county's high school students were recently barred from extracurricular activities for failure to maintain at least a C average, are welcome news.

They are also interesting when cast in the light of the current national emphasis on improving the quality of education. Some other school districts will apparently get the funds they need. Fairfax County public school officials, for example, are confident they will have the money to hire 144 new teachers. The Montgomery County schools plan to add 256 more instructors. More teachers mean lower teacher-student ratios and more individual attention. Unfortunately, the Prince George's schools may have to rely on less than they need. Prince Georges school officials want $2.3 million to add 126 new teachers, enough to lower the student-teacher ratio by only one student per class. The Prince George's system has the highest ratio of students to teachers in the state.

School system officials have also just heard that County Executive Parris Glendening may trim $14 million from the school system's budget request of $362 million in fiscal 1986. That translates into having to make more educational advances with fewer resources than other jurisdictions. This in fact seems to be what the school system has been doing, but there is still room for improvement. Only 38 percent of last year's ninth graders passed a test on citizenship. On a mathematics test that will become a graduation requirement in 1987, 52.3 percent of the ninth graders passed. That is down slightly from last year's class.

With larger classes than they would like, teachers will still have to work harder to help their students improve. It would greatly relieve that struggle if some measure of the millions Mr. Glendening plans to cut could be saved to hire more instructors. They are needed.