The Reagan administration is telling local school districts that they may set up mobile vans on parochial school property to bypass the new Supreme Court restriction against public school teachers entering religious schools to give remedial courses to disadvantaged children.

The practice already is under court challenge in Missouri, and yesterday a spokesman for Americans United for Separation of Church and State -- the administration's main opponent on this issue -- called vans and portable classrooms "a sham, a way to nullify the Supreme Court decision."

In new federal guidelines sent to every state's chief school officers, the Department of Education said that vans or other portable classrooms may be placed on the playground of a religious school or in the parking lot of a religious school, as long as they are not physically adjacent to the school and there is direct access to a public street.

The vans must be a "sufficient distance" from the parochial school building, must be "free of religious symbols" and must not be used for religious purposes.

New guidelines were issued as a result of the July 1985 high court ruling that public school teachers may not enter parochial schools to provide remedial services under the federal Chapter 1 aid program for disadvantaged children.

About 200,000 parochial school children are eligible for remedial classes under the $3 billion program. But since the Supreme Court's decision in Aquilar vs. Felton, local and federal officials have been scrambling to find ways to provide those children with Chapter 1 services without violating the separation of church and state.

The guidelines, released yesterday in separate briefings for reporters, local education officials and representatives of private schools, marked the first time that the administration specifically has backed the use of mobile vans as a way of continuing to provide services under Chapter 1.

The practice is being challenged in Kansas City, Mo., by Americans United. The group is suing school officials for setting up portable classrooms, at times within 10 feet of a parochial school doorway. A trial is set for later this month.

"We feel it's a violation of prior Supreme Court rulings," Lee Boothby, a lawyer challenging the Kansas City school district, said of the administration's new guidelines. "We feel these guidelines violate what the Court has held."

Joseph Conn, a spokesman for Americans United, said: "What's the difference between holding a class inside of a public school and in an annex adjacent to a public school? We see it as just a sham."

Americans United is challenging Education Secretary William J. Bennett in a court case in Washington, accusing him of "foot-dragging" in forcing school districts to comply with the ruling.

Bennett, in speeches, has contested the Supreme Court ruling as "crazy" and accused the Justices of showing a "fastidious disdain" for religion.

In a letter to state school superintendents yesterday, Undersecretary of Education Gary Bauer said that as a result of the Supreme Court ruling, "many eligible children did not receive Chapter 1 services this past year."

A department official said later that about 50,000 students, or about 25 percent of the eligible parochial school students, dropped out of the program because their parents did not want them leaving the school premises for remedial courses or because parochial schools found the new restrictions too cumbersome to continue offering these services.