Oh my god.
there are so many, unfortunately I'm very bad with names.
I'm gonna write short descritptions anyways, so that maybe one of the others remembers the titles/authors.
1. There's one where Buffy's the teacher and Spike's very... well uhm big. hehe.
2. Then there's the Mr Williams series (4 very long stories), it has been put down, but if you ask the author - Pepperlandgirl- nicely, she'll send it to you. It's one of the most disturbing and fantastic stories I've ever read!!!
3. Then there's one where Buffy's a student in boarding school and Spike's the very hot teacher- and he's a friggin priest to boot - very hot. Oh wait, it's on this site and called
Corruption.
4. There's one where Buffy's the teacher and Spike's in a band, but it's not really a high school/college story
See, two titles I could remember, so proud of myself

Have fun!
PS there's also the option to search this site. Just use the summary search and type in teacher or student or something...