It was from season 4, the episode where Washington rewrites Mr. Woodman's song. Horshack wasn't even mentioned in it. I knew Gabe Kaplan and John Travolta missed a lot of episodes but had thought the other three main Sweathogs were in the whole series.

Lawrence Hilton Jacobs is the only actor to appear in all 95 episodes of "Welcome Back, Kotter." Robert Hegyes (Epstein) was absent from the season three episode "Class Encounter of the Carvelli Kind." He did, however, direct that episode. That's probably why he chose not to appear in it.