COLLEGE TUITIONS are rising rapidly at a time

when federal aid to students is about to decline. Any parent with a child in college is all too well aware of it. Who ought to pay--and how much?

Tuitions at the universities in the Washington area have risen by two-thirds, on the average, over the past four years, and that's a great deal faster than the inflation rate. Some of it, the universities say, is an attempt to catch up with earlier inflation that they underestimated. Some of it goes into academic improvements. It's possible to argue that students who can't afford expensive private colleges might better go to the less expensive public ones. The quality of teaching does not differ greatly between the two categories. But the effect would be, to an unacceptable degree, to resegregate the expensive colleges by economic class and, consequently, by race as well. That would be bad for those colleges and for the country.

Anyone who looks at federal aid to students is struck, first of all, by the extremely complex structure of overlapping programs. There are the Pell grants, and the guaranteed student loans, and the supplemental educational opportunity grants, and the work- study money, and more. These programs form a series of compromises--awkward, but more or less workable --between the private and public institutions. The various channels of federal aid to students take account of the reality that a year at one college may cost three or four times as much as a year at another. The Reagan budget cuts are a threat to this complicated balance among many different kinds of universities-- a greater threat to it, perhaps, than to the students who are its putative beneficiaries.

One familiar idea for meeting the steady rise in tuitions is to create larger funds from which students can borrow. But in the absence of heavy public subsidies, that's not a very attractive idea except, possibly, for the professional schools. In regard to undergraduate education, it is hardly responsible policy for colleges to send people in their early twenties staggering out into the world with a bachelor's degree and tens of thousands of dollars in debt at current market rates.

The Reagan assault on student aid is not entirely groundless. There is some reason to conclude that the current rapid run-up in tuitions reflects the common assumption of the late 1970s that federal aid to students was going to be all but unlimited. Student aid seems to have gone circular, with the higher aid in turn inciting still higher costs.

It is obvious that the Reagan cuts in student aid are not going to get through Congress, which is allowing only very modest reductions in next year's budget. But it's also obvious that there isn't going to be the expansion on which a lot of universities were counting. The present rich diversity of American colleges and universities, public and private, is valuable to the country and very much worth preserving. But there is no possible way to accommodate a continuation of tuition increases at the present rate.