NIGHTMARES come early in life and are nothing more or less than a dark, powerful fear galloping across a subconscious that is powerless to object.

I had some pretty awful recurring nightmares as a child, but, thankfully, the most frightening only ran once. That was enough.

It was the first day of school. We were all seated in our wooden chairs in a circle for "Show and Tell." Dewey Stark was announcing that he had new shoe laces. I looked down at my own feet and - eek! - I discovered that I was wearing my pink bunny bedroom slippers. I awoke in absolute terror.

The first day of school is every child's most feared experience, and it can be counted upon to happen every September, like a booster shot, a trip to the dentist, getting lost in a department store. But the first day of school is considerably more fearsome than all of the above because the child's entire life is on the line.

During the three-month summer hiatus, when all the forces for good and evil scatter to their respective homes, one can lay aside one's shield and bivouac in the back yard. But on the opening day of school, the child knows that his or her empire must be rebuilt again - for better or worse.

The hope is that last June' "bosom buddy life-long friend and pal" will have remained faithful to your memory. The dread is that it won't be so. Some terrifying realignment of loyalties may have secretly taken place over the summer. The shiniest new lunch box in the world is no match for that dread.

Diane Sachs, I remember, buckled under it.

Who knows the proximate cause that triggered the reaction; let's hope that it did not seriously alter the flow of her later psychological development. But when Miss Plagaman gave the signal for everyone to stand for "The Pledge of Allegiance," Diane Sachs remained rooted to her seat. Gripping the edge of her desk and crying bitter tears of embarrassment, she endured what all the other kids in the room prayed to God would never happen to them . . .

The most compassionate janitor in the world is not compassionate enough to undo the psychic damage of one such terrible experience, and it would not surprise me to discover that today Diane Sachs is a social worker, still trying to heal-over from that one awful moment in her life. My hat is off to her, wherever she is.

Yet the largest hat must be reserved for my brother, a normal, be-bopper of a California kid who wore pegged chinos, pink suede belts, and played a ukulele.

It may have been the duck tail that he refused to comb out for the Christmas picture, or perhaps it was the fact that he had never heard of Illinois when asked. But my mother found she could not look at him without the phrase "gas station attendant" forming on her lips, and my father wondered aloud whether they weren't making a mistake sending him to a public school that gave academic credit for hay rides. Something had to be done, and something was.

One September morning at the beginning of his sophomore year, my brother found himself 3000 miles away from home squaring up to the front door of a boy's boarding school in Newport, Rhode Island.

Clutching his ukulele and trying to look confident, he was ushered by the headmaster toward a chapel door, behind which the rest of the student body had already gathered for morning prayer.

But of one think my brother felt sure. His suit. The first in his life. It was brown herringbone, had cost a fortune, and was "what they're all wearing," assured my father, who had plucked a yellow button-down shirt and dark brown knit tie off the shelf, written a fairly good check for the whole thing, and marched him into a changing room to put it on.

The door to the chapel was pushed open. My brother, minus the ukulele, was pushed inside.Picking his way to an empty chair, his confidence evaporated into the rafters. Every other boy in the chapel was wearing a charcoal gray suit, with a pink button-down shirt and a black knit tie. Talk about a nightmare -

Yet must must it always be thus? Is there no other way that the first day of school can be approached, even overcome? Is there no hope that children then, now, and forever, will cease to view it as the reverse of their birthday party?

In my own experience, the first day of school was the best day of the year. Unfortunately, the rest tended to be a down-hill slide. Maybe there is no solution, the first day will always be a monstrous gamble. A kid can only hope that sometimes he'll win.

There was a day, not long past, when I walked down the hall with my third-grader son, to enter him into a new school, a new class, with a new teacher. And I can tell you that it was a very long hall with no guarantees at the end of it.

We knocked softly on the door. His teacher opened it and stepped outside. I could hear kids in the class whispering to each other. "Who is it? Will it be a boy or girl?" Necks tried to crane around the open door to no avail.

My son's face was white beneath his freckles and he couldn't have swallowed if his soul had depended upon it. This was the Big Casino and his belt buckle quivered with apprehension.

After a brief moment, during which the teacher solemnly shook his hand, took his lunch box and helped him hang up his jacket, it was time to face the inevitable. He gave me a weak wave, turned and followed the teacher back into the room.

What happened next should happen to every human being at least once in his life. "It's a boy," shouted somebody. "Hooray," shouted somebody else. And for several uncontrolled minutes every other boy in the class cheered, drummed on his desk, and gave the newest member of the third-grade class a totally spontaneous ovation.

My son blushed, his eyes widened in a disbelieving stare, and a grin slowly began to start across his face. It was going to be all right. More than all right.

"That has never happened beofre," said his teacher the next day.

"Once is enough," I replied. And it was.