With college costs rising at more than twice the rate of inflation and with the Reagan Administration's pruning of financial aid, paying tuition bills has become more difficult than ever for many students. And as financing a college education becomes more difficult, many administrators and education lobbyists say they are worried that the changes are curtailing the educational possibilities for both lower- and middle-class students.

While it is clear that college is more costly than ever before, no one is quite sure what effect new taxes and policies have had on the composition of the student population, and what effect they will have in the future. But even though few statistics are available, many observers of higher education say that higher costs and lower amounts of financial aid account for the decline in minority enrollment over the past decade and could lead to further declines.

They also say that the newly cost-conscious middle class students who in past times would have attended a private university might now be inclined to enroll at a less expensive public school--a change that could drive smaller private schools out of business.

"There are a lot of conflicting reports about the effects of what's been happening because there has been no conclusive study. There's a lot we really don't know," says Meredith J. Ludwig, the senior coordinator of research at the American Association of State Colleges and Universities (AASCU).

Some say that the middle class is feeling the worst squeeze from the rising costs. Because of the nature of the changes in the Reagan Administration's financial aid policies, it is members of the middle class, not of the lower classes, who are less likely to qualify for federal help.

"Though the very superior students will always have a multitude of choices because of academic scholarships, the choices middle-class students have are significantly more limited than in the 1970s because federal aid hasn't kept up with the increasing cost of college," says Michael L. Donahue, associate director of admissions at University of Michigan.

"I think that the middle-class is feeling some discomfort, feeling more constrained, because they're having to make more sacrifices," says James S. Miller, Harvard's director of financial aid.

But having to forego a summer vacation is different from having to forsake a college degree, and it remains minority student enrollments that concern officials. According to nationally published figures, both the Black and Hispanic student populations have proportionally declined in the last 10 years.

Black representation in student bodies has fallen from a peak of 7.7 percent a decade ago to 5.3 percent today, according to figures published by Newsweek.

In an unofficial study that has not been released, the United Negro College Fund (UNCF) found that a "small increase in tuition proportionally causes a drop in low income--minority student enrollment," says UNCF Vice President Alan H. Kirschner.

"A lot of it is psychological," Kirschner says. "When newspapers write about increasing costs, students think twice about even applying to college--some don't even bother because it's more difficult to afford. They just go out into the job market."

According to a December poll by the Opinion Research Corporation, the percentage of Americans who think rising costs will put higher education out of the reach of most Americans rose from 77 percent in 1985 to 82 percent last year.

Because minority students are more likely than others to need financial aid in order to attend college, they are hit hard by the combination of decline in federal aid compounded with rising tuitions.

"What has happened is that financial aid money has not kept abreast of the full cost of college tuitions and expenses," says Sarah Melendez, associate director for the Office of Minority Concerns at the American Council on Education (ACE). Because of recessions that have put more minorities beneath the poverty line, she says, today "you have a poorer population trying to pay a higher price with fewer dollars."

Though many colleges award aid to low income students and have developed programs to offset declining minority enrollment, officials say it is precisely the students from underprivileged areas who are less likely to be aware of help available to them and shy away from higher education under the perception that it is a luxury that they can't afford.