Johnny's hair and Jane's jeans are federal cases again.

Their schools -- the federal government has decided -- can't make Johnny wear his hair any shorter than Jane's. And if Johnny can wear jeans, Jane must be allowed to wear them too.

The issue is a more monumental one that it might at first seem. And it had bedeviled the highest levels of government for the better part of a decade. h

Only a year ago, then Health, Education and Welfare Secretary Joseph A. Califano Jr. proposed taking Uncle Sam out of the business of long hair and jeans in schools for once and for all.

With a great deal of fanfare, Califano proposed killing a three-year-old rule against sex discrimination in school dress codes.He called it an example of "unnecessary and inappropriate federal intrusion in elementary and secondary schools."

Yesterday, however, his successor, Patricia Roberts Harris, reversed Califano.

"The discrimination that stems from appearance codes can be as seriously damaging and demeaning as other types of discrimination," Harris declared.

Dress codes, a statement released by her office said, "have been used to enforce sexual sterotyping and to prevent American Indians, Hispanics and blacks from followings customs in hairstyle and dress."

Califano's orginial dress code decision was one of the most popular ones the controversial secretary made during his term in office, and HEW spokesman said yesterday.

"It sounds like you people are beginning to get some sense," said a letter from Tampa, Fla., to Califano after his announcement. "Dress codes are the business of the individual school district. So is about 99 percent of the school's business that you, the federal courts, the IRS and various other people have been meddling in."

"Thank you for freeing up the land of the free," wrote another. "I totally support your effort because our federal government should not be involved in this kind of regulatory measure." $ the rule at that time had been on the books for three years as part of the government's regulations for Title IX, which bans sex discrimination in education. But it had been enforced only sporadically, and HEW's Office of Civil Rights said yesterday that some of the 78 complaints it has on file on the issue date back to 1972.

Most involve male students who were expelled from school for long hair.

The most vocal protest to Califano's proposed rule change, however, came from women's groups.

"This is a situation where local governing authorities are guilty of over-zealousness," said Holly Knox, director of the National Organization for Women's Project on Equal Education Rights. "What schools are doing is denying kids an education because they wear long hair or jeans."

"We usually think sex discrimination only affects girls, but men need to be protected from sex discrimination as well," Knox said.

In one case on file at HEW, an attorney claimed his five-year-old son was expelled permanently from kindergarten because his hair was too long. Knox told of another case involving a girl who wore jeans to school. "The girl was thrown out of school for wearing slacks that were denim rather than corduroy," she said. "This whole thing gets very petty."

In a related move yesterday, the Justice Department charged in a test case that Texas A&M University had discriminated against women students by barring them from the Texas Aggie marching band and three other organizations.

"Every female who has evidenced an interest in the Aggie band has been actively discouraged or dissuaded from joining the Aggie band by band and/or faculty members," the Justice Department said. "other than a desire to keep the band all-male, there is no reason for the current effective restriction of the Aggie band to males only."