ALEXANDRIA'S school enrollment dropped steadily through the 1970s. There were 17,500 youngsters in the city's schools at the beginning of the decade and 10,800 by 1980. Part of it was the migration of families with children to the outer suburbs. Part of it was, for a time, flight from desegregation.

Alexandria had routinely expected that trend to continue, and the schools based their budgets and the staffing on the expectation of 500 fewer children each September. But for the past two Septembers, that hasn't happened. The drop in enrollments has almost stopped. The numbers aren't dramatic, and the effects aren't tremendous. But important changes sometimes take place gradually, and this one is happening for reasons that deserve notice.

One of them is the recession, and parents' reluctance to pay private schools' tuition in times of uncertainty. Another reason for the stabilization is probably, as superintendent Robert W. Peebles suggests, a resurgence of public faith in the public schools. The turmoil and quarreling over racial policy was pretty modest in Alexandria by comparison with other cities, but it had an effect. Eventually it died down, and the change in atmosphere, however slight, may have affected some families' decisions about education -- along with the realization that Alexandria's schools do a good job.

But there's still another reason for the recent surprises in enrollments -- this one more poignant than the others. People who live in this area need to be aware that a wave of immigrants is coming into metropolitan Washington, and most of them are refugees from recent and present wars. Many are from Southeast Asia. The Hispanics now coming into the Alexandria public schools are preponderantly from Central American and, in particular, from El Salvador. There are now more than 120 Afghan children in Alexandria classrooms. Altogether, 12 percent of the children in the city's schools do not speak English as their first language.

In Alexandria, as in most communities, it is the schools and the churches that do most to help the new arrivals find their way. The schools are going to cost a little more than city officials had anticipated, because there are more children in them than the forecasts, based on local birthrates, had suggested. It's money well spent. For these latest newcomers in the classroom, as for all of their predecessors, the public school is the place where they will learn the meaning of equal opportunity.