AT A HIGH SCHOOL not far from here, the principal laid down the rule that this year there were to be no classroom Christmas parties. We are, after all, in the age of Reagan, and the emphasis is on hard work and getting ahead. Classroom parties indicate a frivolous demeanor that is not consistent with the spirit of the times.

Many of the children were disappointed. Their teachers explained that the principal was, of course, correct and that the rule would be enforced. But, the teachers added, on the afternoon before the holidays, there would be a Learning Activity.

This Learning Activity would include foods of all nations, or at least cookies of as many nations as possible, to be brought in by the children. There would be music of many lands, for instructional purposes only, and perhaps some singing. There would be nondirected group discussion.

Christmas as a Learning Activity? The atmosphere in a good high school like this one tends to be strongly competitive. Everybody agrees that competition is good for youngsters, sharpening minds and wills. But once a year, Christmas breaks in with its reminder, to even the least attentive, that perhaps a few other things are even more important.

The full evaluation of the Learning Activity has not yet been turned in. We cannot assure you that each student's manners fully met the national goal --although it seems that they greatly exceeded the national norm. It has not been established whether every kid fully participated in the difficult and unusual exercise of thinking, for a minute or two, exclusively about some other person's concerns and hopes. But most of them appear to have done so.

The measurement instruments currently available do not allow us to tell you precisely what percentage of these students fully grasped the lesson that an hour or two of cheerful conversation, even if not with the most brilliant or handsome of the youngsters in the school, can be a source of great comfort and reassurance in a world that is full of tests and elimination trials. Professional educators will hesitate to accept mere subjective judgments on the matter. But witnesses offer the preliminary finding that the Learning Activity was, as usual, a great success.