There was a collection of his shorts re-published a few years back, and I think some of those were funny. Pushing towards fantasy, anything by Pratchet or Tom Holt is a royal hoot. Some of Daniel Pinkwater's books count as SF, and certainly count as weird humor. I really found funny, however it does deal with the anti-christ coming, so it might not be suitable for a school enviorment. Who with Colin Baker as The Doctor.
Every volume has excellent short fiction, and some of it is quite funny.
The books also include humourous poetry, and the occasional Thurberesque line drawing. Jack Vance's short fiction has some humourous pieces. If you don't want to be restricted to SF, a good amount of humour can be found in P.G. Wodehouse. A single chapter from Wodehouse is usually pretty self-contained and funny. H.L. Menkin is very funny in a satirical fashion.