I've had no personal experience with this USB microphone, but have heard teachers speak positively about it: Blue Microphone's Snowball from bluemic.com.

Mics basically come in two flavors: directional and omni-directional. A directional mic records what is directly in front of it and more or less ignores sounds around the room. An omni-directional mic, on the other hand, records sound all around it fairly equally. This microphone is both omni-directional: good for recording whole class activity, and can be switched to directional.

I am told that the mic plugs into your Mac or Windows USB port and requires no additional software: easy set up and use. For most school budgets, it's probably too expensive for a "class set" of mics. I've seen them: they are not small. I'm told it sounds really great.