When in class, instead of putting cell phone in Manner Mode (vibrate only), put it in Silent All Mode. The vibration function, despite its novelty, makes an extremely faint humming noise and is mildly annoying. (Plus it's not like I'm going to answer the thing.)
ADDENDUM: In retrospect, it was mildly amazing that my cell phone rang in the library. The building is solid stone and I was in a classroom on the floor beneath ground level. Huh. Go Verizon. [11:08 PM]