ONE REALIZES, when it's all over, that high school is a sweet and easy time of life. High school has its struggles, of course; with teachers and sweethearts, with parents and pimples. But these struggles seem, on the verge of later ones, manageable and self-contained. High school is innocence and its end inevitably comes as a shock.

There is a stunning discovery to be made when one leaves. A wilderness exists beyond adolescence, waiting to be tamed. The child must pioneer his own manhood, the future is his whether he likes it or not.

But pioneers are not always brave or wise. As often as not the child makes his most important decisions without knowing what he's doing, or why he's doing it.

This is how George Lucas remembers growing up in his new film, American Graffiti.

American Graffiti is the best and most recent in a long series of nostalgia movies. The time is the summer of '62, the summer of discontent for four Northern California boys who have just finished high school and face the uncertainties ahead with varying degrees of self-reflection.

Lucas looks at 1962 with a critical eye. He attempts a serious examination of the era's inner dynamic. He is not always successful here, but after the corn of The Summer of '42, the effort is appreciated.

The sympathy Lucas feels for his characters transports nostalgia beyond passive entertainment. We are not looking at stick figures in dubious situations. These are people, much like ourselves, going through the same trials that we must face.

AMERICAN GRAFFITI takes place the night before two of the boys are to leave for college in the east. It explores, in a light-hearted and almost superficial way, the traumas of leaving home and the prospect of staying behind. The film's lightheartedness does not disguise importance of the choices each boy must make. Instead, it indicates the way that significant decisions are often made.

Of the two boys who are to leave for college, one begins the night enthusiastic about the new life that waits in the East, while the other is ridden with regrets about leaving home. The enthusiasm of the first, an All-American senior class president dolt, is inertial at best and probably unreasoned.

The boy with the regrets is the most reflective of the four heroes and the most aware of the evening's import. He is also, naturally enough, the one most confused by the choices confronting him.

The two other boys, stay behinds, are both imbued with the local teenage fascination with cars. John Milner, the most sympathetically and carefully drawn character, is the drag-racing champion whose fate is to stay in home town America and race his yellow jalopy until he is finally beaten. Milner, who is the archetypical tough on the outside but soft on the inside hood, is jealous of his two college bound friends, but he accepts his fate without complaint.

Walking through an auto graveyard that contains the wrecks in which past champs have died, Milner becomes aware of his future. He knows one day he will be beaten, but he waits around to take on all challangers. That is what is expected of champs.

THE LAST OF THE FOUR heroes is a jerky guy with no sense of the future. He cries with gratitude when the All-American dolt gives him his car to use while the dolt is at college, and then drives around all night with no particular place to go, trying to impress the girls with his nifty new wheels.

And so it is with the boys -- each facing, one way or another, the prospect of more or less of the same.

But the movie is not unidimensional. Lucas' well written screen-play does not make the boys into symbols pure and simple. Lucas has the courtesy to treat them as flesh and blood.