I was curious to know about the school-related, "slice of life" anime you have seen and enjoyed. I think most (if not all) of these titles are comedies, as such a place would normally make for good jokes.
Pani Poni Dash has to be my all time favorite. It is not only a school-driven comedy series, but a spoof on the genre itself. It's extremely funny as it is extremely fast-paced (not to mention the teacher being the youngest person in the classroom).
Azumanga Daioh and School Rumble would come to a very close second for me.
Eep. I forgot to mention Stellvia: it's more of a drama, but it definately qualifies as a "school" title.