Students will no longer be able to make a federal case over the lengths of their beards or their skirts. The Education Department has issued a final rule, revoking a regulation that allowed students to file federal complaints about school dress codes. It takes effect in 45 days unless Congress acts on it.

The old regulation prohibited discrimination "against any person in the application of any rules of appearance," or, as a department spokesman put it, "school districts could not say boys had to have short hair, if girls could have long hair. Schools could not make girls wear bras, unless they made boys wear bras." When the schools tried to impose such dress codes, students could sue, charging that their civil rights had been violated.

When Education Secretary Terrel H. Bell proposed the change 15 months ago, women's groups opposed it, saying it would foster sex discrimination because dress codes often encourage stereotyped roles for male and female students. They also worried about the symbolic effect of revoking a portion of the so-called Title IX regs, which prohibit sex discrimination in education.

But the department argued, in a notice in the Federal Register, that "this is a matter that should be left to local discretion." A department spokesman added, "This will allow us to focus on more serious issues of sex discrimination, like access to educational facilities, financial assistance and athletics." Bell's action marks the third about-face on the rule. Former health, education and welfare secretary Joseph A. Califano Jr. proposed withdrawing it in December, 1978, but his successor, Patricia Roberts Harris, rescinded that move in November, 1979.