Not quite sure from your post whether you're planning to donate the Christmas trees to someone else or if you'r etrying to get them donated to your school. But here are a couple ideas:

If it's for Christmas trees that will be staying at your school, how about making Box Tops collection sheets that are shaped like ornaments that you can hang on the tree? The kids could have their name on the back and then there could be a prize for the kids who have the most ornaments.

If it's trees that you're donating elsewhere, what if you have the kids trade a BT collection sheet for an ornament they can hang on the tree? It could be a sheet with as many as you want on it. I like groups of 10 b/c they're easier to count! It should be an ornament that they can personalize somehow. You could put a card on the tree that says "Christmas wishes from ___ School" and then all the ornaments with the kids names will be like personal wishes from them to the recipient.

I am trying to think of a contest that the kids could do for Christmas that they could also decorate Christmas Trees and then we could have them donated. I am just not sure how to work it with the boxtops. Anyone, have an idea?