YESTERDAY WAS ONE of the days when I took my son to school. I planned it that way. I wanted to take notes on what his classroom looks like for a column I planned to do about tuition tax credits. It's sort of a cheap shot. My son goes to school in a trailer.

It's not called a trailer. It's called a demountable, which makes it sound like something having to do with horses, but take it from me, it's a trailer.They've painted it white and tan, but it's still made of steel or something and the teacher says that when it gets warm outside it gets hot inside. This is what she told me.

She told me also that there are too many kids in the trailer - more than 30. She told me there are no shades for the windows and that they are running out of what we used to call colored paper and when I went next door to what you would call the real school, they were teaching some kids in the hallway. They had put up a portable blackboard to block the hallway and the kids were being taught behind it.

Outside, some of the swings were dangling from one chain. There was no grass in the yard and the dirt was packed hard as cement. The auditorium is antiquated and when we had a PTA meeting not too long ago, one of the parents had to stand at the portable blackboard and hold it. If you don't do that, it falls over.

I could go on like this, making the point that the public schools could use more money, but that has always been the case and besides the children in my son's schools are learning to read just fine. Anyway, the school I attended was no educational palace itself and we, too, learned how to read - learning even though the place was so old that what was called the New Building went up in 1904. The old building, I think, was there to greet the Pilgrims. The place gave you a weird view of the world, a veiw based on the fact that the gym in the basement has a basketball rim not more than five feet off the ground and a left field fence located about three feet behind third base. It was the only place I've ever seen where a hit over the left fild wall was an automatic out. My school, in short, was a dump.

But it was our dump and everyone went there. Your didn't go if you were religious or very dumb or the child of social climbers. In that case, you went to private shool and your parents paid for it. You got yourself a silly little uniform which may, in the long run, have improved your chances of going to an Ivy League college but which, in the short run, only improved you chances of being beat up.

Things have changed since then and I don't mean to sound like the journalistic version of Sam Levinson - trumpeting the wonder of the good old days. They weren't so wonderful and I'm not so old. But the one thing that has changed is the way we veiw our public schools, the way they are hanging by their nails in some parts of some cities, Washington being one of those cities.

The issue at hand is something called the Roth-packwood-Moynihan-Ribicoff Education Tax Proposal which is not a dream ticket for a New York municipal political race, but a tax credit bill. It would provide that by 1981, a $500 tax credit for every child enrolled in a private school - from kindergarten to college, from Parochial to non-sectarian. There would be no ceiling to this miracle - the more kids you have, the more you would collect, regardless of income. All of this comes a bit late for John D. Rockefeller II who would have, under this proposal, collected $3,000 for sending his six kids to private shools.

The proposed program raised a whole lot of questions. You have to wonder if this is the way to spend the estimated $5.2 billion the program will cost by 1983 and you have to wonder if the part of the program that applies to parochial elementary and secondary schools is constitutional. After all, a tax credit is almost the same as cash in the pocket which is almost the same as subsidizing religious education. Suffice it to say that Griffin Bell, the attorney general of a president who himself campaigned on a plank of helping private education, thinks the program would be unconstitutional.

I leave these matters to the fiscal and legal experts and return to where I began - the neighborhood school of ours. It needs a lot of things, but like a lot of Washington schools and a lot of schools elsewhere, it needs more middle-class and upper-class kids - white kids, black kids, who-cares-what-color kids. It needs, in short, the sort of kids who are now going to private schools and it needs them lest the public schools turn into a preserve for the children of parents who can't buy their way out.

Anyway, there's nothing particularly myterious about all this. It's a mix. You look at the schools and you look at your bank account and you do, you hope, what's best for your child. There's hard cash in that mix and for lots of parents, that's enough to tip the balance toward the public schools. Given $500 those parents could go the other way. Given this program and a trailer for a classroom, they just might.