THE PURPOSE of attending a college reunion is to demonstrate that you have learned something since you left. What you wish to demonstrate having learned--professional or romantic success or perhaps just competency--depends, of course, on what your classmates thought you least likely to succeed.

Some of you may not have found time to study for the reunion in the last five, 10 or 50 years, however, and so Miss Manners is going to cram you with some last-minute information. She will include a few facts for those who are not actually at the reunion for credit, but who, as spouses or children of classmates, or as graduating seniors of the institution, are voluntarily or involuntarily auditing the event.

The first principle is that the rules of bragging in polite society apply at college reunions. Anxious as you may be to replace your undergraduate reputation with the improved version you have since devoted yourself to developing, it cannot be done instantly. Information that is pulled out of one is not only more socially acceptable than the kind that is announced or displayed by its possession, but it is more readily believed.

To show that you are now rich and important, for example, you do not show up flashing expensive things. (The proper dress for the occasion is a refined version of whatever the school's faculty of your age group tends to wear.) You describe yourself as "working for" an organization of which you are the president, for example; you explain the position, when questioned, as "managerial" or "administrative;" and you only admit to the title on the third round. Or you arrange to be harassed by calls from your office requesting decisions during this little holiday, or to look fretted to distraction after checking the financial pages of the local paper.

To seem newly attractive to those who refused to date you in college, it is best to have an accomplice in the person of a tender and solicitous spouse. Recently acquired spouses, especially for those a few decades out, used to be considered particularly good for this purpose. But nowadays, to appear with an original spouse who seems still in love with you is more impressive.

Bringing a date to a college reunion is a big mistake. Perhaps you have forgotten what a dormitory critique is like. If you don't have a spouse, it is better to have passionate telephone calls.

The other chief principle has to do with how to treat others. Warily. You don't know whose fortunes have changed, especially if the others are following Miss Manners' rules, and the class butt may your boss's hero be.

Temptations to confess your old secret estimation of others should be resisted. Sentences beginning with "You probably never knew this, but I thought you were . . ." should not be finished. Or started. Miss Manners doesn't know which is worse--confessing an old crush (with the implication that it seems silly now) or confessing an old animosity (with the implication that you now forgive the offensive quality or think the person has outgrown it).

You realize, of course, that the worst thing you can do is to re-create your collegiate behavior. But, then, that is the fun of college reunions, isn't it?

It is the duty of spouse and children to support whatever claims are made or postures taken--and some of them will be strange--and to bathe the reunioner in a glow of love and admiration from those who obviously know him or her now. The spouse, after all, probably has a college too; and the children may want to get into the institution themselves, some day.

As for the graduating seniors who may be forced to witness the foolishness of alumni revisiting their old haunts--which may be those seniors' living quarters--restraint is required. Yes, it is irresistibly comic to see middle-aged people pretending to have been the same age once as oneself, but it is wise to resist laughing in their faces. Miss Manners will be glad to tell them why if they will come back and ask in five or 10 years.

MISS MANNERS RESPONDS

Q. It was recently suggested to me that I should consider a van or a pick-up truck as an alternative to the station wagon (known to one of my origin as a shooting brake or estate car) which I proposed to buy. I take the view, however, that a gentlemen should never be seen at the wheel of those types of vehicle. My wife adds that a lady should not even be seen dead in one; in fact, especially when dead.

Apart from my problems of nomenclature, is there any etiquette recognized by persons of propriety in connection with informal vehicles?

A. Your wife is quite right that the correct vehicle to be seen dead in is long, low and black, while vans and truck tend to be high and bright.

That, of course, is a formal vehicle. But one has to be a person of considerable propriety, as it were, to maintain separate vehicles these days, for occasions of varying formality. Ideally, one uses a car of a cheerful color, durable and roomy inside, for daytime, and for evening, a black car with soft upholstery that does not damage sliding silks or furs. However, if one has to choose, it is more proper to attend a party in a bright car than it is to make the children run alongside a sleek car to school every day, because you are using a chic evening vehicle that doesn't accommodate their requirements.

But we were talking about informal vehicles. Miss Manners sees nothing distasteful in the domesticity suggested by a van, or the rusticity associated with a truck. She is only puzzled as to how they got into your letter, when neither you nor your wife wants to have one.