WHY SHOULD the parents of school-age children vote against a proposal for education tax credits? The Standing line of opposition to the tax credit proposal, which is being considered for the District's ballot in November, is that it will badly damage the public schools. Even so, you have to ask, why should a parent put the abstract idea of the good of the public schools before the well-being of his own child? If education tax credits will enable a parent to get a better education for his child, then, self-concerned as it may seem, every parent could be expected to do what is best for his child -- and who could argue? But there is faulty logic here. For education tax credits will not enable most or even very many parents to procure that better education, but they will undermine the possibility of giving children a good education through an improved public school system.

The education tax credit being considered is not large-enough to help most parents afford a private or parochial school if they cannot afford it now. At $1,200 per year, that tax credit would represent some slight relief for people who can already afford the cost of sending a child to a parochial or private school.

Parents will find that schools vary in quality and that private schools, like most other services sold on the market, vary sharply with their cost. At a tuition of $1,200, or anything close to it, a school will have little capacity to deal with the individual differences of individual children. As children depart from the average, they become more expensive to educate. The high costs are associated with the extremes of ability and disability. Public schools generally have the recourses to deal with a range of differences. Private schools, except for those with substantial tuitions, generally do not.

If education tax credits are approved, they will take an estimated $20 million to $30 million out of the city Treasury every year, and most of that money is sure to come out of the school budget, chipping away at the public schools' ability to teach bright or slow children and to afford special programs. Approval of the tax credits is also likely to be seen as an expression of lack of confidence in the public schools and will prompt more parents to take their children out of the schools, further lessening pressure on public officials, principals and teachers to improve public schools will not be there as a fallback option for parents who find private schools disappointing.

Even for parents who want to keep their children in public schools but see the tax credit as money for extra public school programs, there is a pitfall. The money for the extra program will come out of the same city Treasury that provides for the school budget. Any added money for after-school programs will mean money for regular school programs.

Instead of allowing the public schools to be underminded in hope of being able to afford the perfect private school, most parents would do better to work to improve the public schools. The District schools are on the right track, establishing new promotion standards and clearly defined objectives for what children should be learning in every day of every grade. An academic high school for the brightest students and a good new superintendent hold even more promise. For all but the children of the very well-to-do, the public schools are a better bet than is the dream of getting a tax credit that will pay the cost of private education.