Who hands out the proof order forms to the correct parent? Do you take them back to the teacher to pass out? Mail them to the parents directly?
I contract for a large sports photography firm (photo mill

), and we have bar codes on each order form that must be filled out prior to shooting, either by the parents, or if they're not there then incompletely by the child himself. He at least knows his own name, and usually a phone number. We shoot the order form (the equivalent of putting your hand in front of the camera), then 5 shots of each kid (I'd do only 2 or 3 for static portrait events, 4 or 5 for action events).
On the back end, the software reads the bar code and sorts the following 5 photos into that kid's online account. The parent logs in, orders, and the photos are shipped directly to their home.
To scale it down, just have the teacher write the child's name on the order form and put a number instead of a bar code like A1, B1 as suggested, and take a photo of the kid holding it. But every kid gets an order form that's ready for proofs attached whether they're prepaid or not, completely filled out or not. If they're not prepaid, the order form can go back to the school for the teacher to pass out with the proofs attached as suggested. If prepaid the parent already filled everything out like their address and email.
What about renaming the files just to something like 20091104classa001.jpg 20091104classa002.jpg ? Then you'd only have to change the name at each new class?
If it's a small shoot I can handle it w/o an assistant, but otherwise bring somebody to write the numbers down and handle money if it's paid that day! At the sports firm, there are 3 of us: photographer/tech, action poser, and parent customer service form fill out assistance :)