Man, I love both these shows. I approached them expecting yet another stupid harem/pervy show, but both of them really paid off by approaching it in a rather adult fashion, dealing with some pretty real problems realistically.
I thought that Twins stretched the realms of belief more than Teacher (well, for a show where one of the main characters is an alien), but I don't think it really detracted much from the show. Both really captured the awkwardness of juvenile romance pretty well.
I was rather disappointed in the soundtracks, but the OP/ED and image songs are great....