We give our teachers a voucher at the end of each school year to buy items they need for their classroom. When they make their purchases they submit their receipt(s) and we reimburse them.
We only issue one check per teacher. They do not submit their receipt(s) until they have reached the amount.

I wouldn't want to allocate the money up front. As you said, sometimes teachers do not use the whole amount.

In recent years our PTO has allocated a sum ($300 per classroom in recent years) to cover supplies that would otherwise come out of teachers' pockets. We have required teachers to submit receipts, and then paid out up to the $300 sum. This year I'm wondering whether it would be easier to just pay out the $300 at the beginning of each year to the teachers, under the assumption that most of them will use it up anyway.
ADVANTAGES: Administratively, it's just one check per teacher and less paperwork to track.
DISADVANTAGES: Some teachers don't use the full allocation (or at least don't claim it); that has always given us a little surplus that we carry over to the next year.

Any thoughts on how to do this? Do you pay one sum or require receipts for reimbursement?