Tenth-grade students in Arlington, Loudoun and Prince William counties scored well on the state minimum compentancy tests that measured reading and mathematical skills, according to figures released yesterday by area officials. Of the nearly 1,100 10th graders who took the tests in Loudoun County, 90.3 percent or 987 students passed the reading and 93.3 percent or 1,011 students passed the math test, said Arthur Welch, a school official.

In Prince William County, where 2,660 students took the tests, 90 percent or 2,398 passed the reading test, while 90 percent or 2,404 passed the math, school spokeswoman Joyce Hart said.

Of the 106 newly enrolled 10th graders who took the tests in Arlington, 86.7 percent passed the reading test and 86 percent passed the math, according to school spokeswoman Joan Lewis. Only newly enrolled 10th graders were required to take the tests because Arlington school officials devised their own minimum competiency tests last year (prior to the state tests) and gave them to their ninth graders, Lewis said.

The state minimum competency 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 earning course credits.

In figures already released for 10th graders in Fairfax County and the city of Alexandria, 90 percent and 80 percent, respectively, passed the tests.