THE SUPREME COURT'S decision to uphold, 5 to 4, Minnesota's tuition tax deduction is a poorly reasoned and unpersuasive interpretation of the constitutional separation of church and state. Minnesota's state income tax allows parents to deduct their children's school tuition and fees up to $700 a year. That provides, indirectly but very effectively, a state subsidy to private schools.

Justice Rehnquist's majority opinion reads like a result in search of a rationalization. It will encourage a proliferation of tax expenditures to finance private schools--and how about college tuition?

On the legal points, it is difficult to decide when government tax or spending policy stops being broad support for charitable activities and becomes support for religious activities. But the majority's effort to distinguish earlier cases striking down various grant and tax schemes doesn't work. They argue that the Minnesota law applies neutrally to both public and private schools. But this apparent neutrality ignores the actual effect: 95 percent of Minnesota's private schoolchildren are in sectarian schools, and deductible costs at free public schools are negligible. Although the purpose of the statute may be secular--support for education--the effect is to direct these tax dollars overwhelmingly toward religious institutions. Once they are there, the commingling of religious and nonreligious activities is unavoidable.

The court also says that Minnesota has used a "genuine tax deduction" similar to other credit and deduction provisions and that such legislative choices about tax burden are entitled to judicial deference. But this is not a complicated policy choice among different classes of corporate taxpayers or different kinds of tax shelters. The majority's further argument that Minnesota's financial support is indirect and only as a result of parental choices about schooling is a fair distinction from outright grants to religious institutions, but it hardly balances the reality: rather than mere "attenuated financial benefits," religious schools stand to profit handsomely from schemes modeled after Minnesota's.

The court's constitutional decision does not change the merits of the policy arguments. Tax benefits for private school tuition represent a threat to public education, not healthy competition. Public education has been and remains a crucial community institution and a vital aid to social and economic advancement for the great majority of families. It is unfortunate that just when the country seemed on the brink of a major new commitment to good public education, the court has encouraged a divisive diversion.