Educational vouchers represent an idea whose time apparently has come -- again.

The question was debated a decade ago, and at least one modest experiment was undertaken -- in Alum Rock, Calif. Then the debate subsided and the issue went back to sleep.

It's wide awake now. John E. Coons and Stephen Sugarman have written a book urging adoption of a voucher system, "Education by Choice." Milton and Rose Friedman devote to vouchers an impassioned chapter in their book, "Free to Choose: A Personal Statement."

Catholics, whose parochial schools are feeling a financial pinch, are arguing for vouchers. A referendum on the subject was defeated last year in Michigan, but a similar proposal will be on the California ballot in 1982.

It's a safe bet that you'll be hearing a lot about vouchers in the months ahead.

The basic idea is fairly simple. Take the per-pupil cost of educating children in the public schools and give that money, or some fraction of it, to parents in the form of a voucher with which they can then purchase their children's education. If parents are satisfied with their public schools, they will keep sending their children there, their vouchers being used to pay the cost of operating those schools. If parents are dissatisfied, they can take their vouchers to the school of their choosing: another public school or a private or parochial one. s

The advantages, say the backers of the idea, include equity, increased parental influence and improvement in the quality of education.

Equity because parents who send their children to non-public schools pay twice for their children's education: once through their taxes, which support the public schools their children don't use, and again through the tuition they pay at the schools they do use.

The parental influence comes from the fact that vouchers would be the lifeblood of the schools, public or private. Where parents have no choice as to where their children will attend school -- as is the case with most low-income parents -- the schools feel no necessity to be responsive to their desires regarding discipline, course offerings or quality of instruction. Vouchers would force them to compete and, therefore, to respond to the needs of the consumers rather than to those of the educational professionals.

And since the schools that failed to meet these needs would go out of business, the net result would be an overall improvement in the quality of education.

Good teachers, with the guarantee of a clientele able to pay, would be free to start their own schools. The choices already available to affluent parents would become available for the first time to poor parents. Racial and economic integration would be enhanced, because it would be voluntary, not forced.

Nor, say the advocates, is this the end of the advantages. As Coons says in a recent Newsweek column: "Children who attend public and private schools picked by their parents simply do better. They learn. They enjoy learning. They are more tolerant of individual difference. And this pattern holds irrespective of family income. Private schools in the slums turn out educated children."

But wouldn't a voucher system (or its much-debated kid brother, tuition tax credits) serve to destroy the public schools, already fighting for survival?

Say the Friedmans: "If the public school system is doing such a splendid job, it [need not] fear competition from non-governmental, competitive schools. If it isn't, why [should] anyone object to its 'destruction'?

"The threat to public schools arises from their defects, not their accomplishments."

The case for vouchers is so strong, say its supporters, that they are at a loss to understand why so many people outside the public education bureaucracy are leery of it. In particular, they profess not to understand why some of the most leery are low-income minorities, whose children, in their view, would stand to gain most from the choices vouchers would make available.

There are reasons for these misgivings, and these will be discussed in a subsequent column.