Education Secretary William J. Bennett yesterday criticized school-based birth-control clinics as an "abdication of moral authority," saying that the clinics "legitimate" sexual activity while encouraging teen-agers to have "sexual intimacy on their minds."

"Birth-control clinics in school may prevent some births -- that I won't deny," Bennett said. "The question is: What does it teach, what lessons does it teach, what attitudes does it encourage, what behaviors does it foster?"

Bennett's remarks, in a speech to the Education Writers Association in Baltimore, were disputed by The Planned Parenthood Federation of America, a strong proponent of school-based birth-control clinics. Planned Parenthood's local affiliates have pioneered such clinics in some school districts.

David Andrews, executive vice president of the Planned Parenthood Federation, said Bennett's view "goes back to the old argument that ignorance is better." He said, "Ignorance unfortunately has caused more unwanted pregnancies than any other factor.

"Unwanted teen-age pregnancy exists, whether we like it or not. And given that it exists, the question is: What is it that we as a society can do," Andrews said. "To believe that somehow one individual's moralistic view is the answer to teen-age pregnancy is just kidding yourself."

Teen-age pregnancies in the United States number about 1 million a year, twice the rate of teen-age births in Canada, England and France.

The number of school-based birth-control clinics -- usually offering comprehensive health services -- has proliferated, but the estimated total is still less than 50 nationwide. Wednesday night, the Alexandria School Board decided against establishing a health clinic at the T.C. Williams High School, which would have been the first in this area.

Bennett said he was giving an individual opinion, since the decision to establish birth-control clinics on school premises is up to local school boards.

But the secretary, who during his 14 months in office has been an outspoken advocate of morality in American schools, said establishing birth-control clinics represented a "classic bureaucratic response" to the teen-age pregnancy problem that was "highly questionable, if not offensive."

He said such clinics give young people the wrong signal, by showing adults "acknowledging as commonplace what ought not to be commonplace."

"This is obviously a local decision," he said. "But I would say this to any locality considering it -- you had better be sure, really sure, that you have consulted fully and thoroughly with parents. Otherwise, you may find that you have created a full enrollment policy for private schools."

Andrews said birth-control clinics based on or near schools are supported by teen-agers, their parents and by a 78 percent majority of Americans in an August 1985 poll commissioned by his group.