SCHOOL SYSTEMS have to be tailored to the communities they serve. Alexandria has developed an unorthodox pattern that sends all the city's students to one big school, T. C. Williams High School, for their last two years. This winter the school administration has raised the question whether to split that school into two conventional four-year high schools. The answer, at a series of public hearings, has been an outpouring of support in favor of one central senior high. That support is well deserved.

Wide variations in a student body always place special demands on a school system. Alexandria's secondary-school students are an approximately equal balance of white and black, with a substantial Asian minority. Because of housing patterns established in the days of legal segregation, it has always been hard for middle-class black families to find homes in Alexandria; most of the city's black children come from families that haven't much money or much education. But in a lot of the white families, the parents are highly educated and anxious that their youngsters compete successfully for admission to good colleges. There are some poor white families; there are some Hispanics. To neglect the interests of some categories would bring litigation; to neglect others would invite white flight to the outer suburbs and to the private schools.

Alexandria used to have three four-year high schools, each reflecting its own constituency. Where the proportion of college-bound students was lowest, the choice of academic courses was narrow and tough luck for the talented youngster who wanted to take calculus or a fourth year of French. The bestequipped vocational shops were at the newest school, and students at the others knew that they were getting second best. The school administration was able with a bit of gerrymandering, to see that the racial balances in the schools were all about the same. But there was no possible way simultaneously to balance all the courses offered. Some of the students began to charge -- quite legitimately -- that the disparities amounted to a denial of equal opportunity.

The solution was to send all of the city's 11th and 12th graders to T. C. Williams. There everyone gets the same access to a very wide range of programs. The vocational training is specialized and effective. In science and mathematics, in which the school has earned a wide reputation, able students can advance into college-level work. The school offers five languages. Because of its large student body, it can provide a breadth of instruction that none of the conventional schools previously could have matched. Perhaps the average child does not gain much more from T. C. Williams High than from its predecessors. But the school proves itself in educating the children who are not average -- the very bright, the handicapped, those whose ambitions or needs or interests are exceptional.

The Alexandria School Board has not yet reached a formal decision, but it clearly does not intend to split up the city's senior high school. That's the right choice. T. C. Williams High has become a model school for a highly diverse community that believes in public education.