The Justice Department continued its reversal of Carter administration school desegregation positions yesterday, but stuck with the previous federal stand in a key sex discrimination case.

In a brief to the Supreme Court, Rex E. Lee, the solicitor general, and William Bradford Reynolds, head of the civil rights division, said the department no longer believes a Washington state law barring a voluntary busing plan in Seattle is unconstitutional.

The 9th U.S. Circuit Court of Appeals ruled last December that the state busing ban did violate the equal protection clause of the Constitution's 14th Amendment.

Camden M. Hall, an attorney for the Seattle School Board, said yesterday he considered the Justice reversal "unethical and a breech of legal canons" because the city had given the department lawyers confidential information that was now compromised. He said he was considering filing a motion to prevent Justice from participating in the case.

"It makes the whole process look pretty shabby," he said. "I'm sure the Supreme Court will recognize it as the raw political maneuver it is."

About 10,000 of Seattle's 45,000 students are being bused under the voluntary plan, according to a School Board lawyer.

In another brief filed late Tuesday, however, Lee and Reynolds continued the previous Justice position of defending regulations that said Title IX of the Civil Rights Act of 1964 provides sex discrimination protections to school employes as well as students. In doing so, they rejected the Department of Education's recommendation last month that the position be abandoned.

The new filings are the latest in a flurry. Late last week, the solicitor's office said the government no longer took a position on whether Texas had to provide free education to children of illegal aliens.

Lee said in a telephone interview yesterday that he thinks the Reagan administration's position on the three cases "shows we're applying the principle that this department is not a captive of any political point of view. Simply, we're going to do what is right and follow what the law dictates in each particular instance."

The department's new stand in the Seattle case continues a trend in school cases. In the last few weeks, Justice has accepted a school desegregation plan for Chicago that it had rejected just a month before. And Reynolds decided not to press a federal judge in Houston to force desegregation between the city and its suburbs.

In the brief reversing the Seattle position, the Justice officials said they had "reconsidered" and no longer found "controlling" the two Supreme Court precedents it cited before.

Instead, they cited an opinion in a Dayton busing case. " . . . It is undisputed that the state of Washington has ultimate authority over public educational policy," the brief said. "There is thus no proper basis for concluding that the action taken at the state level was any less valid than the court of appeals apparently concedes it would have been if taken at the local level."

Lee said he thought "it was perfectly proper to take a fresh look" at the Seattle case and consider the Reagan administration anti-busing philosophy because the Supreme Court precedents were split.

Iris M. Green, the civil rights division attorney who worked on the previous Seattle court papers, said last night that she was never consulted about the change of position. She said the new Justice stand "is ridiculous and absurd. Our thinking was corrent and well-reasoned. The state law is still unconstitutional."

In the Title IX case, Lee said he considered only the legal issues. He was opposed by womens' groups at his confirmation hearing, but said he felt no pressure to act to please them.

The case arose when the then Department of Health, Education and Welfare had threatened to cut off funds from two Connecticut school boards for alleged discrimination against employes.