College tuitions will rise an average of 7 percent this fall, the College Board said Friday. Students at the most expensive private schools can now expect their four-year education to cost upwards of $80,000.

Average "fixed charges" at private, four-year institutions - tuition, fees, room and board - will hit $11,330 for 1988-89, up 9 percent from last fall, the board estimated in its annual survey of college costs.Adding another $1,600, which the board estimates the average student spends on incidentals like books, supplies and transportation, the total budget for resident four-year private school students will reach nearly $13,000.

"We are, of course, not at all surprised by these increases," said Bruce Carnes, deputy undersecretary of the U.S. Department of Education and an outspoken critic of rising tuitions. "We have stated that so far as we can see the price of college is going to go up at this rate forever."

Average fixed charges at four-year public schools will go up a more modest 5 percent to $4,445 for in-state students - or just over $6,000 including incidental expenses.

At two-year private schools, tuition, fees, room and board will average $7,627, up 7 percent. Average tuition and fees at public two-year institutions are increasing 4 percent to $767.

Students at tiny, selective Bennington College in Bennington, Vt., can expect costs to reach approximately $20,590, counting estimated incidental expenses. Sarah Lawrence College is second most expensive at an estimated $20,360, followed by Brandeis University, $20,186, and Barnard College, $20,150.

Other schools where student expenses will hover near the $20,000-a-year mark include Tufts University, University of Chicago, Harvard and Radcliffe, Dartmouth College, Boston University and New York University.

"Students and their families should not be discouraged by these increases," said College Board president Donald Stewart. About 5 million students will share more than $24 billion in public and private financial aid next year, he said.