Ok...I'm no expert here by any means, however from what you're describing, it doesn't sound so much like a "Mac" problem as much as just different settings somewhere between your systems at home and those at school. I will say that yes, it seems Macs can be a bit more twitchy when it comes to some of this stuff...things that open and run fine here at home on my PC's will error out on the Macs at the college I attend and my humble PC renders out faster than those Macs costing more than 4X more. In this case however, it almost sounds like perhaps the model you downloaded has some instanced geometry or something of that nature. Is this a character model perhaps? Instancing is VERY common with character models as well as cars or anything else with semetricle geometry. Perhaps for some reason your Macs at home aren't seeing the instanced geometry...although I can't really say why (maybe check your preferences and such).
If this should be the case...a character or car or something else of that nature, what you could try is open the model at school, delete the half of it that you're Macs aren't seeing, then use duplicate special to re-create it from the other side (without instancing). Do save it as a new/seperate file so you don't mess up the original, but this way you can see if this is indeed the problem.
Likewise, I would also check to see if part of the model is by chance referenced as well. Because Macs and PC's have a different directory structure, if that missing half of the geometry was by some chance referenced, your Mac at home may not be seeing the reference. In theory this should blow an error asking you where the file is, but with Macs, ya never know.
Okies...hopefully someone with more experience will come along, but until then I hope this helps!
