Pretty much all digital cameras in the past couple of years fit the bill. I'd recommend a Canon SD100. Anything in the Canon SD line is good, but that's the first of the line, so most likely to be available used (no point paying full price for something that the kids may steal or smash). 3MP, 2x zoom.

Of course, if you bought something current, it'd probably have 3x zoom, and more resolution, so that you could get a finer picture even at a distance. If you intend to take still pictures in the school, I recommend Canon over Kodak and Nikon, which have terribly weak flash.