Thanks to Ken & Mike (and Bob, of course) for the great info.
I just ordered a:
TAMRON SP AF 17-50mm F/2.8 XR Di II LD A16
from eBay (new for ~$350). I found out that A16 is the "non-VC" version so I looked into that a bit. It seems like "non-VC" is good enough for me (I just need to try to have a steady hand or lots of light)... it should arrive within a week and I'll get to play! There are 4 more field trips this school year so I'll get to test them out quite a bit.
I'll need to wait on the Canon 85mm... don't want to spend all my money in one go
Might be a good birthday gift for myself though!