Secretary of Education Terrel H. Bell has asked the Justice Department to switch sides in a pending Supreme Court case involving sex discrimination in education -- much to the dismay of civil rights advocates.

The issue is whether the law prohibiting sex discrimination in educational institutions (Title IX of the Civil Rights Act of 1964) extends to employes as well as students. The Carter administration had argued in court cases that it does.

If Reagan's Justice Department agrees to go along with Bell, some civil rights activists contend, it will be a clear sign of the direction the administration plans to take on discrimination matters.

"No facts have changed here. The legislative history has not changed. The only thing that has changed is that an election was held last November," said Judith Lichtman of the Women's Legal Defense Fund. "Our concern is that this is a matter of law, going before the courts. If it is a matter of politics, then it ought to be fought out in the legislature."

James Corey of the U.S. Commission on Civil Rights said that his office has written a letter to Bell "to express our concern."

A spokesman for Bell, confirming that he has contacted the Justice Department on the matter, said, "In a careful reading of the statute, we don't think the law justifies such an expansive interpretation" as the previous administration had given it.

The Justice Department declined comment.

Since the mid-'70s, school district officials have challenged the federal regulations on this subject, saying that they exceeded their authority. Civil rights interests, backed by the government, lost in a long series of court cases, in which the judges agreed with school officials that the law does not extend to employment cases.

But recently, two appeals court decisions upheld the government regulations, creating a conflict that the Supreme Court has agreed to review.

Sex discrimination in employment is prohibited by other parts of the civil rights act and an executive order.However, Lichtman said, the Title IX regulations shifted the burden of enforcement from individuals, who must sue for their rights, to the government by providing that federal money could not support programs that discriminated.

The change would affect the individual teacher or coach who believes she has been discriminated against, according to the Bell spokesman, but the protections would still extend to certain employment cases where the discrimination is deemed to affect students -- for instance, where there is a pattern of not hiring women that denies the students proper role models, or where the case involves students employed in a federal program.