Cressida, are you looking for a model kit for high school classes? Depending on your level of familiarity with molecular modeling, here are several computer programs you can utilize for this purpose without paying huge $$:
1. Check ChemDraw by Cambridgesoft - it is a chemistry drawing program and an add-on to it allows 3-D renedering of organic molecules, both space-filled and ball-and-stick that makes molecules look like those plastic kits. There should be a free trial download available and academic pricing, too.
2. Ages ago, when I was in grad school, I bought one of the first versions of HyperChem (may be even the first version) - a neat 3-D modeling software for Windows-based PC (in those days, most programs for modeling run on very expensive Silicon Graphics computers). HyperChem > Home ( DNN 4.3.5 )
HyperChem is still around, and now it can be run on a Mac, too. It might be a bit too complex for what you are trying to do.
3. I had some good experience with ACD Labs' software in the past, too. I have not used it in years, but it might fit the bill perfectly for your use since it is supposed to be very simple to use.
4. The plastic modeling kits can be obtained from Aldrich or any other scientific supplier: Molecular Models MolyMod molecular models
Frankly, I have not touched a plastic model in ages, so I can not be of much help here.