THE BRIGHT fall air smells like leaves, and all the small clouds sailing in the sky make blots of shadow along the ground. The kind of weather that makes you nostalgic for whichever one of you 16 school years it was when you walked across a green campus shaded by dying trees, with 1,800 pages of the world's greatest literature under arm and an absolute confidence in your government, your teachers, your illimitable sexual future . . . and football.

What could be better than, than, picking out some weekend full of football weather and going back to school - for a game? It dosen't have to be your old campus - it could be any of the dozen schools within an hour or so's drive. Pick the kind of school you remember best - but for the most fun, pick a small college, the kind of place with a pretty good team that has never ever sent a time completely, to the wooden bleachers, the girls from the service sororities selling football chrysanthemums, tailgate picnics with lots of booze. A gang of 10-year-olds celebrating their permission to be rowdy until gametime by practicing intricate broken-field running on the bright grass behind the end zones. Real grass on the field. Players who are the size and shape of human beings. A light wind putting a healthy flush on everybody's cheeks and the cheerleaders' thighs . . .

AT LEAST that's what you'll find if you are sensible enough to check the weather first. I didn't. And so I wound up at Franklin and Marshall College in Lancaster, Pa., on one of those fall days of the other kind: Clouds so low you could almost reach up and touch them, a sharp little wind and a stubborn little half-rain, the kind that always seems just ready to stop, and just ready to start up again when it finally does stop. A miserable day.

And actually, as I realized a few minutes after I got out of the car, only another kind of good football weather. The girls selling pompoms were wrapped in nylon slickers; the kids did their broken-field running in fancy parkas with the names and colors of NFL teams; the band played loud - louder when the rain squalled up again. Bring along your old sweatsuit top with the hood, an army surplus poncho (or an umbrella if you've come to that), slide a pint of very forbidden alcohol under all that covering . . . what could be better? The cheerleaders cheer in the rain, curls sticking against their cheeks and temples: "Eat 'em up, beat 'em up, rah rah rah!" I jin in, warm and dry, discovering that I have not actually said "rah, rah, rah," since junior high.

FRANKLIN AND MARHALL is playing Johns Hopkins, a team that does not figure to give them much trouble, says the guy in his early 30s besides me, well-wrapped - and very well-lined - with his own weatherproofing. He is my running commentary during their first names, complaining at every gain because it isn't long enough, always trying to outguess or predict what the coach will do - and he is always utterly wrong. He is not what I thought he was - an old grad.

"I'm back in school. Oh, I dropped out before, you know - the Democratic convention in '68 . . . The Democratic convention. I was there - Woodstock. Now I want to be a lawyer. I had all that stuff. You know I think back on Woodstock - all that rain, and mud . . . and horribleness. It wasn't any fun. It wasn't any fun! Ohhhh, look at that!"

Franklin and Marshall is having a little trouble with Johns Hopkins - the ball is slippery and Ed Turner, F&M's fullback, has fumbled.

"Eddie's not getting that ball tucked away and he's in traffic! Two hands, Eddie," he screams, "or you're going to pop it again. They gotta keep the ball away from him," he says as F&M intercepts a pass and begins marching downfield. "You'll see. You see? You see that! You! See! That!? What a player!"

EDDIE HAS just scored a touch down and my friend Woodstock is explanning that he knew it was going to happen - "All you have to do is give him the ball and he'll run it. Good old Eddie! Rah, rah, rah!"

He is much more convincing ar "rah-rah!" than I am, but I am getting so that I'm pretty good. Of course, we have the experience: The words may be slightly different, but my friend and I have been chanting and cheering like this for years - ever since the Gulf of Tonkin.

Back to the game. It is a very nice game. No players carried off on stretchers, very few penalties. One personal foul seems a result of overanxious officiating - a quarter-back slips on the wet grass and a guard tumbles over him. On the Johns Hopkins side, the cheerleaders look just like F&M's wet and smiling girls in different-colored costumes made by the same tailor, and they go through the same traditional arm motions and jumps in celebration of every first down and completed pass. Except that they have fewer people to cheer along with them, and less to cheer about. F&M keeps lengthening its lead, and finally wins 30-14. Opposing team captains shake hands and slap butts. The sun suddenly shines down a little gray light, and the rowdy kids take over the whole field to themselves, an impressive game of touch.

HOODS OFF, umbrellas down, the 300 or so people who watched the game to the end walk to the gates. There are so many male undergraduates with sort hair that it looks like the '50s - and so many women with wedge haircuts that you can't tell the girls from the boys.I would like to complain about that - for old times' sake - but instead I introduce myself to a long-haired bearded man in his 50s who turns out to be an ex-deans, he seems to have that self-congratulatory machiavellian attitude of a successful campus politican who never grew up. We have a long talk about the time he managed to sneak the Dow recruiter past the five members of the SDS chapter who had bought a pig and were parading it around campus with antiwar signs.

"Things are easier now," he says, with a voice full of regret. I know how he feels. Things are easier with me, too. That's why nostaligia is so popular - it's the only way we can permit ourselves to envy our own past.

If you want to see some of these good little games nearby, or nearly nearby, Saturday's your day. This Saturday, F&M will be playing at Widener College, in Chester, Pa., Georgetown will play at Johns Hopkins in Baltimore and Towson State's Tigers will go to Bowie State to tangle with the Bulldogs there. Check with the schools for exact times, and keep an eye out for the old school spirit. It's still around. Rah!