WALKING OUT of UNESCO a year ago, the United States pledged to keep open the possibility that this fallen institution might yet regain the value to make it worth the American while to rejoin. To this end the Reagan administration posted an official watch and set up a heavyweight citizens' commission to oversee the adventure of reform. It continued consultations with the 20-odd Western countries that -- most of them -- were scarcely less disturbed than Washington by mismanagement and politicization of the United Nations Educational, Scientific and Cultural Organization but which had decided to use the shock imparted by American withdrawal to test the chances of reform from within.

So how are things going? The British had said they'd quit by the end of the year if . . . They were a bit vague on the if, the better to allow room for maneuver on an issue where the political community is split, journalism is engaged and Britain's European allies and Commonwealth partners are pulling it in different directions. At the big biennial UNESCO conference just completed in Sofia, the British hedged, joining the prevailing consensus on key resolutions but inserting reservations. The resolutions had to do with areas where UNESCO's fuzzy leftism has in the past sent Western democrats up the wall. One such area is international communications -- controlling the media is the familiar UNESCO itch. Another is "people's rights," with which some UNESCO folks would like to replace human rights. It was, for the West, an uphill struggle.

There was, all the same, some movement at Sofia. Resentment at the American withdrawal was tempered by a strain of regret and hope that the United States will reconsider. Disputes over the position of the American observer mission, American financial obligations and the rights of American nationals on the UNESCO staff came out in a way satisfactory to Washington. Some progress was recorded in budgeting and management.

And although the United States has insisted it won't get personal, it escaped no one's attention that the Soviet bloc suddenly withdrew its support for a third term for secretary general Amadou Mahtar M'Bow, who lost the confidence of most Westerners years ago. The Soviets apparently feel that a club without American members isn't much worth belonging to.

The United States left UNESCO because the organization was standing its founding ideals of freedom on their head. Working to reform UNESCO is the best tribute to the role it might again play.