THE FIERCE QUARREL over school closings in Montgomery County draws attention to the rapid social changes overtaking the Washington suburbs. There are now large and growing immigrant populations and strong patterns of ethnic diversity. For the Washington area, that's new. As a southern city, Washington is accustomed to thinking in terms of only one ethnic distinction, between black and white. As for economic class, Montgomery has generally been regarded as the least heterogeneous of suburbs. But the reality now is quite different. As always, the schools bear the primary responsibility for answering the needs of the newcomers. That's why any reorganization of the system, like the one in which Montgomery is now engaged, requires precise attention to the meaning of that statistical category labelled "minorities."

Out of the 96,000 children enrolled in the Montgomery County schools, some 7,100 are not American citizens. Most of these children do not speak English as a first language, and many hardly speak it at all. What do they speak? The list runs to some 80 languages.

Among the children counted in the statistics as belonging to minority groups, barely half--12,200 children--are black. There are also 6,300 Asians and 4,100 Hispanics. The county follows the federal government's definitions of minorities, which greatly understate the complexity of the new demands on the schools. This sytem counts Iranian and Arab children, for example, who may have little or no English, as part of the white majority.

This diversity is by no means peculiar to Montgomery County. Every school system in the metropolitan area is dealing with it, and some in greater degree. But, as you read the references to minorities in the Montgomery school debates, these figures offer a reminder that the term is not merely a code for black children and the familiar issues of racial balance in the classroom. It's become more complicated than that.

These figures also suggest that, without much notice, an important new responsibility has devolved onto the public schools. They now have the crucial job of teaching English to another generation of newcomers and, since the language is the carrier of the culture, of providing these children with their first reliable contact with American life. Historically, it is a function more familiar to schools farther to the northeast than in this region of the country, and more familiar to the central city schools than to the suburbs. When school budgets come up next year in Montgomery, and throughout the area, the various boards and councils will need to keep it in mind that another duty has been added to the schools' agenda. In this country, the ability to speak English is not what you would call a frill.