BY NEXT FALL, the cost of a year as a residential student at a four-year private college will average about $5,110. At a four-year public college, the average will be $3,050. The College Entrace Examination Board has published its annual survey of tuitions and living costs and, to no one's surprise, the figures keep inching upward. But the interesting thing about the current rise - 6 percent from this year to next, the College Board says - is that it's just about the same as the inflation rate. Or, to put it the other way around, when you take out inflation, you discover that the real cost of going to college has not risen over the past decade.Meanwhile, the amount of financial aid available to students - scholarships, grants and low-interest loans - is currently expanding very fast. This year, the increase will be in the order of 15 percent.

One question of public policy here is whether middle-class families are being increasingly and intolerably squeezed by the cost of sending their children to college. Is the pressure getting worse? The evidence shows that, in most cases, it is not. Costs, since the late 1960s, have not risen as fast as average incomes. Equally important, the widening flow of student aid is now directly helping families well up in the middle-income range. The College Board has now become a central clearing house for financial data, and it points out that even families in the upper-middle income range are now eligible for substantial amounts of help. Precise amounts vary with families' circumstances. But the aid requests submitted to the College Scholarship Service for the current year showed that families with incomes as high as $35,000 a year are sometimes eligible - especially if that family has more than one child in college.

On the average, College Scholarship Service figures show, a family with an income between $25,000 and $30,000 was expected to contribute about $3,400 to student's college costs this year. The student can usually earn another $800, for a total of $4,200. For all costs over that figure, this average family was eligible for aid - $800 if college costs were $5,000 a year, $2,800 if they were $7,000 a year. It's quite true that federal aid tends to be channeled to youngsters from poor families, who need it most. But the rising amounts of federal help for these students is leaving other funds - aid from the states and the colleges' own scholarships - available to students whose familiest stand higher on the income ladder.

These figures reinforce the advice that colleges give to those high school students - most of them - who worry about money: Apply to the colleges that you want to attend, without regard to tuition. Then, if you are accepted, work with the colleges to see what aid, in one form or another, might be provided.

These figures also offer one more good reason for Congress to abandon the mischievous idea of a tuition tax credit. It isn't needed. The present system - a mixture of federal, state and private funds, with varying purposes and conditions - is working. The tuition tax credit is based on the erroneous idea that the load on families with children in college is getting steadily worse. On the contrary, there has never before been so much aid available to students - nor has it been available to families over such a wide range of incomes.

One refrain frequently recurring in current American politics is that the middle class is feeling bruised and a bit sorry for itself. All the tax breaks go to the rich, all the benefits go to the poor and - according to the indictment - all the bills go to the families in the middle. College costs usually turn up high on the list of examples. But in this case, at least, the example actually points the other way. College costs are going up no more than the general rate of inflation, while student aid is increasing much faster. For most families, including those in the middle-income range, the burden of paying for college is now beginning to come down a little.