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.

Lucas uses the pursuit of adolescent sex as a device to pace the film and develop his characters. The dolt must separate from his high school sweetheart, the captain of the cheerleaders. He discovers that he can't. The one who is reticent about leaving home spends the night pursuing a boss blond, more dream than dreamy, who is tooling in a white T-bird. Milner, a lonely fellow at heart, wastes half the evening trying to get rid of a junior high school girl he has picked up by mistake, and spends the other half learning to appreciate her company. The jerk is able to pick up a good looking blonde because she thinks he can buy her liquor, and he occupies himself by trying to win the girl with phony stories of wealth and possession.

THE PRESENCE AND IMPORTANCE of the girls makes the story work and is necessary in the development of American Graffiti's theme. None of the boys can see their way to the future alone. Their only vision of what will happen to them, of what they want to happen to them, is the reflection of themselves they see in the girls.

American Graffiti is not a soft soap film like its nostalgia predecessors. The end of the film, which is a mite too fanciful (with the college bound boy flying away on "Magic Carpet Airways"), is sober if not sobering. Everybody does not get his girl. But everyone does make his decision. One way or another, for one reason or another, the boys have taken their cards and Lucas will force each one to play them out.

American Graffiti is not just a sequence of events. It is a nostalgia movie a la mode and its success in reconstructing the early sixties is phenomenal. From the Coke to the burger stands with waitresses on roller-skates, from the souped-up cars to the ducktails and pony-tails, from the gang of hoods to the sock-hop, Lucas is unerring in his eye for detail and in his ability to bring his audience eleven years into the past.

Graffiti's collection of cars makes the movie a must for any greaser or reformed greaser. The driving on the strip raises cruising to the level of ballet.

LUCAS' BEST TOOL for recreating the early sixties is the soundtrack. From beginning to end the film is accompanied by sixties golden oldies. The silly lyrics and uncomplicated melodies put you in the appropriate mood faster than any ten reruns of Leave It To Beaver ever could.

The acting in Graffiti is sufficient. The female leads are exceptional in their ability to remind us what it was like to grow up female in the sexist sixties. Ronny Howard, who played Sheriff Taylor's son on the Andy Griffith Show, gives the film's only unsatisfactory performance as the dolt.

American Graffiti is not a profound film, but it's especially well-done and a lot of fun. It is a memory, perhaps a memoir, of what one goes through on the way to adulthood. If you're one of the freshmen registering today, you're probably wondering if you should ever have come. American Graffiti will comfort you with the knowledge that you are no different than anyone else.