Q. "My son is 11 and in the fifth grade.

"We've recently moved here from the Midwest, and I find that sex education will be part of his science class next semester -- and for this we're paying a whopping tuition at a private school.

"I think this is outrageous.

"He is much too young to be thinking about sex. Television has made children think about it years ahead of time and this so-called 'biology class' will only make matters worse. It could turn his mind into a sewer.

"I want to bring it up to the parents' association, but I dread standing up and talking about sex. Maybe I could do it if you would give me some ammunition."

A. An 11 is too young to have sex, but you can bet he wonders about it and watches every 13-year-old nymphet who sashays by, even while he stubs his shoe and pretends to whistle so he won't have to say hello.

This is normal, and it would happen no matter the influences. Certainly there was no television around to make Romeo and Juliet act so sexy at 14. And only last week we heard of a splendid child who is secretly writing a dictionary of dirty words. Although it has no definitions (since she doesn't know what the words mean), her friend is writing one that does and it's going to have pictures. And yet these children are happy, normal 8-year-olds -- who don't have a TV.

It's a lot healthier to be curious about sex than to pretend it either doesn't exist or -- since it does -- that it shouldn't. The urge to reproduce is one of the most basic instincts. It can and should be controlled until it is appropriate, but it can't be ignored.

It does, however, need to be talked about.

Even if most parents could discuss sex easily with their children -- and few of us can at first -- it still should be part of elementary education.

When your child is ready to learn geograpy, you expect it to be taught. And when he is wrong about a fact in history or math, you count on the teacher to correct him.

Sex is a fitting subject, too, although human reproduction is only part of the story. It is the major part, but it should be taught in the context of all reproduction, including plants and animals. Leaving people out of a study of biology would be like leaving them out of a course in world history.

But the taboos -- and the urges -- of sex do make it a dicey subject, so the teacher has to be good. She should be comfortable enough with the subject to talk about it in a straightforward way, and mature enough to not rely on jokes or playground vernacular. This not only cheapens sex and embarrasses the children, but if they don't learn to be easy with the clinical terms they won't know how to talk about it with other adults -- like parents.

Your son also needs a teacher who can explain to him that his feelings -- even at 11 -- are normal and that they will get stronger in the next few years and be more difficult to curb, particularly until his conscience has developed as well as his body.

Moral development, like desire itself, is gradual. That's why your son needs to know about the consequences of sex as well as the process, and he needs to know now before the urges get too powerful.

He needs to know that there are 1 million teen-age pregnancies in this country each year: about 13,000 involving girls younger than 14. The father may be a little (or a lot) older, but no matter the age, the male is as responsible for the pregnancy as the female. Your son has to learn that if you're old enough to have sex, you're old enough to prevent conception, if not by abstinence then by contraception.

And if you think your child has never thought about birth control, it's time to think again. Go into any big drugstore and you're sure to see a pre-teen trying to read the fine print on the contraception display from five feet away. tHe's got to get the information somewhere.

A child also will feel more in tune with the consequences of sex if he has a chance to know small babies.

Rather than attack the school for having a class in sex education, you can offer to broaden its scope by suggesting that the children study babies after they're born, as well as before. It would be a great way to put sex in perspective and to learn that every child is irreplaceable.

Although it wasn't part of a sex course, the Collegiate School in New York -- a private school for boys -- has taught baby care to fifth and sixth graders and has found it extremely successful. The results are in a charming paperback of quotes and pictures: "Oh, Boy! Babies!" by Alison Gragin Herzig and Jane Lawrence Mali (Little, Brown and Co., $5.95).

Even if the school won't add this class, your son -- and any boy this age -- would profit by the book. By teaching child care so young, the class caught the boys before they had learned to hide their sensitivity behind the macho mask so many men wear. With luck, perhaps they never will.