Four of every five 10th graders in Alexandria schools passed thestate's new minimum competency tests measuring reading and mathematics skills, according to figures released last night. School officials saidthe fact that 20 percent failed was not discouraging.

"I am encouragedthat 80 percent passed the tests," said School Board Chairman Alison may. "Now the board will be looking at the remainder of 20 percent to seethat they pass the test. That's our job ahead."

The results promptly generated a sharp disagreement between school officials in Alexandria and neighboring Fairfax County, where fewer than one out of ten 10th graders failed the new tests, according to results released earlier this week.

Several members of the Alexandria school board said the Alexandria results should not be compared with the results from the county.

It has come to my attention that some of the school systems, especially one that has received a lot of publicity lately, didn't follow the state guidelines," said board member Claudia Waller, referring to Fairfax County.

Waller said some handicapped and foreign-born students were excluded from taking the test in Fairfax County.

"We tested all our 10th graders," she said. "Fairfax County chose to do otherwise. (their figures) are not and honest score of the schools system."

Fairfax County School Superintendent S. John Davis vigorously defended the results for his schools last night, calling the assertion that Fairfax failed to follow guidelines "ridiculous." He said only students who recently arrived in U.S. and are unable to read or write English were excluded from testing. He said these numbered fewer than 50.

In Alexandria, the tests were administered at the two schools that enroll 10th graders-Francis Hammond and George Washington high schools.

According to the figures, 79.9 percent of the 10th graders at Hammond passed the math test.

At George Washington, 78.9 percent passed the reading test, while 74.6 passed the math test.

Students who failed the minimum competency tests will be allowed to take them again in the spring and every succeeding year until they pass, said Donald E. Dearborn, associate superintendent for instruction. The test must be passed as a prerequisite for graduation.

The tests were mandated in a 1978 law requiring all students, excluding the graduating classes before 1981, to pass state tests measuring reading, math, social studies and job entry skills, in addition to mandatory course credits.