The Supreme Court's approval of substantial tuition relief for the parents of private school students will have a social impact that can only be described as mean, since it not only deals a blow to the future of public education in our country, but also undermines one of the basic tenets of our democracy.

No matter how the court attempts to dodge it, the arithmetic of educational financing in America dictates that in helping private schools, public schools are automatically hurt. Educational funds won't grow like wild mushrooms--there is a limited universe of financial support for education. The Supreme Court has given approval for that limited universe to be divided in a new way--subsidizing private education tuition. This necessarily robs public schools of badly needed resources and points toward the ultimate undermining of public education in this country.

It discriminates against public schools because there is no equal opportunity for parents of public school children to use the tax deduction. Justice Thurgood Marshall was right when, in his dissent, he wrote, " . . . Parents who send their children to free public schools are simply ineligible to obtain the full benefit of the deduction, except in the unlikely event that they buy $700 worth of pencils, notebooks and bus rides for their school-age children."

The right to an affordable public education has been the key to making our nation a republic, and thus one of the fundamental tenets of democracy. Insofar as public schools are compromised, then, this aspect of democracy is compromised as well.

Not long ago, a national commission on education issued a scathing report on U.S. schools: America, it said, has squandered a post-Sputnik surge in the race for knowledge and committed "an act of unthinking, unilateral educational disarmament" that threatens its very future. It called for tougher standards, longer school days and higher pay for teachers to combat a "rising tide of mediocrity."

While the commission's report clearly gives us reason to worry about our public schools, the Supreme Court's decision gives those who wanted to avoid public schools anyway a means of paying their way around the bad schools.

As America becomes increasingly worried about competing with the Russians and the Japanese, some may see providing a good education as a patriotic duty. These same people may believe that the only way to get a quality education is through private schools, and the court decision paves the way for private schools to be subsidized by the federal government.

Thus, at a time when we ought to be concentrating on the good of the whole, this decision diverts attention to special interests that might spring up for the most narrow reasons. This may mean, for example, that parents who think schools should teach one way can set up their own strange enclaves and support them with tax money. Now religious schools or people who just want to get their children away from busing can find a constitutional blessing and financial aid.

People who are wealthy with power and money may find great comfort in this decision, but the non-elite who are already in a bad way will be in a worse way still.

It is important, too, that blacks in particular realize that this decision adversely affects even those who can today afford private schools. They should not allow themselves to be misled. Most blacks earn their incomes from services provided to other blacks. If the people who patronize the black doctor or businessman are being discriminated against, that person will ultimately have less money to spend, and the person who can afford private schools now may ultimately suffer as well.

But this is the deeper issue for America: If you don't intend to educate people fairly in a society that is becoming increasingly technocratic, what do you intend to do with them?

When this court decision is taken together with other trends, such as the growing emphasis under the Reagan administration that affirmative action means reverse discrimination, the way is paved for setting back causes that America has traditionally stood for. It bodes ominously for our future.