THERE'S NOTHING WRONG with cutting federal aid to college students -- so long as the cuts are aimed at the excesses in the wide-open, heavily subsidized loans. That is the Reagan administration's intention, and it's on the right track.And yet its attempts to make the cuts quickly are holding up the commitments for next fall and threatening to tie up even the most justified and badly needed help for students. The struggle with student aid is an instructive example of both the need to get a handle on the budget and the practical difficulties of actually doing it.

The federal government has two major channels of financial help for college students. One, the basic opportunity grants, is based on families' incomes and the students' willingness to help themselves through part-time work. It's an extremely valuable kind of scholarship assistance that allows talented youngsters of very limited means to attend even the most expensive colleges.Here Mr. Reagan proposes only minor adjustments.

But the other main route for aid, the guaranteed loans, is a different matter altogether. Some of these loans are serving a useful purpose. Many of them -- and no one knows how many -- clearly are not. The cost has been rising like a rocket. In 1978, $1.6 billion was loaned. By 1980, the rate of lending had tripled.

Why so fast? Because in 1978, in an attempt to deflect the campaign for tuition tax credits, Congress and the Carter administration dropped all the eligibility requirements for these loans -- which, you realize, are heavily subsidized. For anyone who began to borrow before the end of last year, the interest rate is still 7 percent. Even for new borrowers, it will be only 9 percent.

These loans are available to the family of any student, regardless of its circumstances. If parents have the money on hand to pay college bills, they can take out a loan at 9 percent and redeposit the money in, say, a money market fund at 14 percent. Who makes up that fat 5 percent of easy and effortless profit? The taxpayer, through an off-budget mechanism that the Reagan administration is now going after. It's true that the president proposes to throw a lot of people out of the loan program. Most of them should never have been let in.

Unfortunately, it's not only the upper-income sharpies who are going to be penalized. The next few months are the time of year when students arrange their opportunity grants and their loans. But both are now frozen while the administration tries to put through the changes that it wants in the eligibility requirements. That creates great uncertainty and confusion among students for whom it will mean the difference between returning to college next fall and dropping out. In the subsidized loans, the middlemen are the banks -- and most of them are now getting very nervous about commitments to lend under rules that may shortly be changed. While many of those loans have been frivolous, some are essential to students who now find the banks reluctant to deal with them. It appears that, as usual, the rain is going to fall on both the just and the unjust -- and perhaps, next winter, the snow as well.