Q. How does one help a "squirmish, fidgety, can't-sit-still-for-a-minute" child achieve in school?

My son will be 6 in a few months. He has an excellent vocabulary, quick mind, good memory and is eager to learn. He attended kindergarten last year and seemed to enjoy it, but had trouble learning because his attention span was never more than 15-20 minutes.

But here it is, the third day of first grade and the teacher casually mentioned that he seems "restless and fidgety."

I should point out that there were rather drastic changes in our household last year: His father and I separated and my son and I moved to a neighboring state. His father calls and sees him, but not frequently. My son seems to have adjusted fairly well to these changes, although over the summer there were more than the usual number of fights with neighborhood children and too many displays of "baby-machismo" activities -- boasting and bragging and breaking house rules.

How does a parent (and his teacher) help a child who is a ball of action most of the time but who sits still for television, listens to a story and plays with toys that require little mobility?

Incidentally, his recent physical check-up showed him to be quite healthy with no eye or ear problems.

A. There is a two-part problem here.

A child who is months younger than most of the children in his class usually has a tough time in first grade, especially a boy, since boys mature about 6 months slower than girls.

Last year was a tip-off of trouble ahead. A healthy child who is ready for early kindergarten should be pretty comfortable in first grade, unless the academic standards of the new school are much higher than the old one -- which wouldn't be apparent on the third day of school.

It might be wise to put him in a less structured class -- an easier first grade or even a kindergarten class. Sixth grade is the classic time when early starters are held back, but it would be much less traumatic to have him repeat a grade now.

Your child's need to slow down now has nothing to do with his ability to learn, but his maturity. He's only 5 years old, and with the changes in his household, he's hugging hard to what used to be. That "baby machismo" last summer is right in tune with a 4-year-old's behavior.

Which brings us to Part II.

Divorce is enormously hard on children and you're seeing some of its toll. Aggression is fairly typical in the child whose parents have parted: He's mad at the world and takes out his anger on friends. He feels abandoned and he doesn't know why -- except that at 5 he is so egocentric he's sure he is the cause of it all, which only makes him more upset.

There are two books that might help you understand how older, more articulate children look at divorce, so you can see how confusing it must be to someone his age. One is The Kids' Book of Divorce, edited by Eric Rofes and written by 20 children of divorced parents between 11 and 14 at the Feyerweather Street School in Boston (Vintage, $4.95). It can be ordered from Box 1000, Brattleboro, Vt. 05301, with an extra dollar for handling.

The other: How to Live With a Single Parent by Sara Gilbert (Lothrop, Lee and Shepard, $6).

For your feelings, there is Single Parenting by Stephen L. Atlas (Prentice-Hall, $5.95), which is, as it claims to be, "a practical resource guide."

Or you might attend one of the free mental-health workshops on "Single Parenting" 7 to 8:30 p.m. next week around the city. Co-sponsors are the Psychiatric Institute Foundation and the C & P Telephone Co. For more information, call: 467-5637.

All the workshops and books in the world, however, may not be enough to guide your child. Because it's hard for a parent to be objective about her child in the best of times, he may need a psychotherapist for a few months. This will help him explode in a more constructive way, without feeling guilty for getting mad at you.

This doesn't mean he's disturbed, any more than a retrenchment at school means he's dumb. Your little boy just needs time and t.l.c. to unload the heavy freight he carries, and he needs it without having to respond to heavy pressure at school.

Learning to read is the least of his needs now.