THERE ARE those who are still wringing their hands over the imminent American departure, now confirmed, from the United Nations Educational, Scientific and Cultural Organization, and over the very idea that the United States would pull out of an international body. We are not among them. UNESCO had it coming. A certain effort has been made to characterize withdrawal as a cheap bone the administration was throwing to the far right. But this always underestimated both the extent of UNESCO's failings and the breadth of the political community troubled by them.

The capture of the organization by Third World- communist elements fundamentally opposed to its founding and still-worthy purposes was well established. The management of UNESCO had made of it a boondoggle. Perhaps, as some suggest, the administration was not deft enough in its attempt to promote reform from within. But reform still was, and is, essential. Jimmy Carter reluctantly took the United States out of the International Labor Organization, continued working to improve it and then went back in: a positive precedent.

In fact, a reassuring thing happened on the United States' way out of this forum. From acting as though it were determined to bash the international community in the face, the Reagan administration went a good way toward committing itself to secure the further changes that would justify and obligate an American return. So its announcement that it will indeed withdraw at the end of December, as it warned a year ago that it might, is not a negative step, or at least it need not be.

There is a difference, in Elliot Richardson's phrase, between being a dropout and being a reformer. The American effort to reform the organization from within did not work well enough, but the administration promises to continue that effort from without, and others are watching closely to help it along that path.

This leaves only the M'Bow problem. In giving final notice, the United States said it does not deal in "personalities." Perhaps it can't. But the personality of UNESCO Director General Amadou Mahtar M'Bow of Senegal is a key ingredient in this stew. More than any other single person, he made UNESCO the distracted forum it is. If he had decided some time ago that the best way he could serve it was to leave it, the organization might not be in its current difficult straits. He has a heavy responsibility for what happens next to a body that has done valuable work in the past in culture, education and science, and that can do valuable work again.