Sorry to point out the elephant in the room but do you really need the extra 0.5GB RAM that 64-bit will give you on a 4GB machine? Also, have you considered that a lot of older curriculum titles may fail on 64-bit OS when they worked fine on 32? I'd stick with the image you have and see if anyone notices - I'd put £5 on the fact they won't!
Once they start getting machines with 6-8GB then you need to move, but you can plan now in slow time, perhaps over the summer?