The increase in financial aid to scholarship students will be reduced by an average of $200 next year as a result of the Ivy League presidents' decision Wednesday not to follow revised College Scholarship Service (CSS) guidelines.

The guidelines adopted by the presidents call for an increase in financial aid over this year, but a smaller increase than suggested by the CSS.

"There was general agreement that the old tables were unfair to parents," John G. Kemeny, president of Dartmouth, said last night. "But many schools felt they just could not afford the new CSS formulas."

William J. Raduchel, director of freshman scholarships, said yesterday the Ivy formulas are fairer than the CSS tables but "fairer does not mean cheaper," he said. "We believe the guidelines are at a level families can afford."

A family paying $1350 under the old guidelines would pay Harvard $1150 under the Ivy League guidelines, while the CSS guidelines would have asked for only $900.

Kemeny said, "If we're overgenerous with one student, we'll be unable to afford fewer financial aid applicants."

Others Are Interested

The "Seven Sister Schools," including Radcliffe, will go along with the Ivy presidents' decision. Raduchel said other colleges had expressed an interest in the Ivy guidelines, and he expects many schools to adopt them.

CSS guidelines are used voluntarily by colleges to compute the percentage of a student's expenses that should be paid by his family. Harvard generally makes slight changes in the guidelines before applying them, Raduchel said, but rarely makes as widespread adjustments as this year.