The Reagan administration has passed up a second opportunity to challenge a court ruling that restricts the government's ability to enforce laws against sex discrimination in schools.

The Justice and Education departments let a 14-day deadline pass without asking the 6th U.S. Circuit Court of Appeals to reconsider its decision that Hillsdale College in Michigan need not promise to comply with Title IX of the 1972 Education Act amendments. Title IX prohibits sex bias in federally funded education programs.

In an opinion similar to one handed down last July by a federal district court in a case involving the University of Richmond, the appellate court ruled, 2 to 1, that Title IX extends only to school programs directly receiving federal funds, not to institutions as a whole. Since the only federal funds entering Hillsdale are destined for student aid programs, the college argued, any regulations on its use applying to the school as a whole are invalid.

Civil rights groups had urged the government to ask for a rehearing of the Hillsdale case, noting that one of the judges died before the opinion was issued, making the ruling a 1-to-1 vote that carries no legal weight. But despite the storm of protest that followed its decision not to appeal the University of Richmond case, the administration let a Dec. 30 deadline pass without requesting a rehearing.

"The choice was a knowing one," the National Women's Law Center said last week. "It is clear that the government lacks any real commitment to civil rights enforcement."