Howard Hesseman left because the show focused less on him and more on the students. Tannis Vallely was written out because puberty was taking its toll on her. As for Leslie Bega and Jory Husain, I think that the former left by free will to pursue other projects, and the latter was written out because his character didn't "add" much to the story (because they didn't work his character enough, in my opinion!). Am I right? Who knows the real deal?