Short summary: the new substitute teacher is a giant preying mantis who eats virgins. The end.
Analysis: Ah, the first clunker. This episode tries to give us another monster-as-metaphor, but ends up just making us confused. Teachers are...bugs? Stay a virgin or die? (Well, "sex equals pain" was a lesson Buffy taught well.) There is a sort of enjoyment in watching a bad episode, though, as at least you can have a good laugh.
In any case, the praying mantis teacher was one of the few clunkers of the first season. The only good parts about this episode is the increasing wordplay the characters use and the subtle use of Angel. There wasn't even much to analyze.
Note: my DVD was much better on this episode, which is weird because I wouldn't have minded if it did skip. There were times when I wished it would skip.