The Prince George's County school system - the only one in the Washington area to test prospective teachers in spelling and grammar - is considering becoming even more selective by adding a basic arithmetic test.

The proposal, which was to be considered at last night's School Board meeting, was prompted by the decision of D.C. Teacher's College last month to allow two students to graduate even though they failed required courses in mathematics, according to Sue V. Mills, the school board member who drafted the testing proposal.

The spelling and grammar test now administrated to job-seeking teachers "weeds out" more than 20 per cent of the applicants, county school spokesman John R. Aubuchon said.

The math test proposal has the conditional support of the county teachers union - the Prince George's Educators' Assocation - although the union has opposed it in the past.

According to spokesman Steve Bittner, the association voted recently to support the testing "because teachers are flooding the market and it is probably appropriate that we are more careful in selecting teachers."

Two conditions of the association's support, according to Bittner, are that the test must not be given to currently employed teachers and that classroom teachers must be involved in preparing the test.

School Supt. Edward J. Feeney described the current teacher hiring situation as a "buyer's market." He added that the county is looking for teachers specially trained in dealing with the problems of low income children and in teaching basic reading.

Feeney said he and school board members have asked the University of Maryland to make such training available.

Carl McMillen, the school system's director of personnel, said the spelling and grammar tests, which have been given to more than 2,000 prospective teachers since 1975, "are one more step to insure the quality of new county teachers."

The number of applicants who fail the "simple test in grammar and spelling," McMillen said, shows that "colleges are not doing the job, so the school system must make sure teachers are qualified."

Civil rights groups which generally have opposed teacher tests, charging that they discriminate against blacks and other minorities, have not made that accusation against Prince George's County's tests, McMillen said.

"These tests are for basic spelling and grammar, which all applicants should have knowledge of," he said.

As reported earlier this month, District of Columbia schools plan to implement teacher testing by 1979.

City School Supt. Vincent E. Reed said the test, if approved by his school board, will be a written examination in the fields applicants want to teach. Reed is also proposing that new teachers be given oral examinations and be required to demonstrate their teaching abilities through supervised teaching or submitting a video-tape of themselves teaching in a classroom.

The D.C. school system stopped testing new teachers in 1969 and since then has used applicants' college records and interviews for evaluation.

The only other school system that tests new teachers is Montgomery County, which has given a test in English grammar and usage to prospective English Teachers. The test has been given for the last four years and about half of those who have taken it have failed.

According to Montgomery school spokesman Kenneth Muir, the school system has not tested other job-seeking teachers in other fields and does not plan to because of difficulty showing that the tests are job-related.

"A physical education teacher, for instance, could tell the school system that while it was important to know English grammar he would not need it to teach his students," Muir said.