THE DEPARTMENT of Health, Education and Welfare ruled the other day that the public school system in Wethersfield, Conn., can have an all-male choir if it wants to without endangering its federal aid. So what else, you may well ask, is new? Well, the fact is that that is news, thanks to the same Department of HEW that, during one of its zany periods last summer, ruled just the other way.

The issue of all-male and all-female choirs got somewhat lost at the time because of the warnings HEW also issued against father-son and mother-daughter functions at public schools. While those warnings were withdrawn quickly once they came to the attention of President Ford, the choir question persisted. In fact, Wethersfield disbanded its choir rather than risk the loss of its funds. Now, however, the flip-flop is complete. Wethersfield's sixth grade boys can sing together without the girls and without endangering the money as long as the school system has an equivalent choir for the girls. The ruling by HEW says it has concluded, somewhat belatedly we suggest, that there is a reasonably clear non-sex distinction between members of these two kinds of choirs - a difference in the "range or quality" of voices.

We never understood why HEW found it necessary to get into these matters in the first place. There are enough major problems of sex discrimination in education without singling out what are - even with the greatest stretch of the imagination - only fringe questions. Neither choirs nor parent-child functions were among those practices Congress intended to reach with the anti-discrimination provision of the 1972 education act. By fiddling around with them, as it did last summer, HEW created the impression that it didn't know the difference between things that really do and don't matter. For the sake of effective enforcement of the rest of its anti-discrimination regulations - the serious part - we hope that HEW now knows the difference.