Hi John,
A school's budget may be better spent getting several small printers that multiple students can use vs. one large printer. As print size goes up, print time increases exponentially, and it's best to not leave printers running unattended, so a bigger build area isn't necessary good for a school setting.
As far as important features, you want to look for a printer that doesn't require proprietary filament, or you'll end up paying extra unnecessarily over the long run. Most printers can use generic filament - some exceptions are Cubify and newer Makerbots - stay far away from them!
Filament is usually PLA or ABS. PLA is corn-based, stiffer, better suited to figurines, and has less odor. ABS is stronger and more flexible, more heat resistant, good for making functional parts.
Dual print heads are more of a gimmick than an actual useful feature.
An enclosure and heated bed are nice, especially if you want to print with ABS plastic. This is because ABS will warp while printing if there is too much change in temperature from the ambient air or from the plastic cooling after being printed.
I would recommend the Ultimaker if you just want one printer, or several of the Printrbot Simple Metals if you'd rather have a small fleet.
You may also ask Mike Hathorn if you'd like advice from a teacher using Sketchup and 3D printing in a classroom setting.
