Dear Nathan,
How about buying the Christmas trees as container grown trees, and setting up a system where your pupils look after the trees all year in containers, and you charge a fee to parents or anyone interested to hire the tree over the Christmas period. It's very environmental and carbon neutral.
You can charge the fee over and over again so long as you keep the trees alive, and it could even make a profit!
I know of a farm near Bolton in Lancashire that do it, I think they charge £15 per tree with a further £10 deposit to be returned when the tree is brought back. Security may be a problem, but it's worth a thought, as it could be up and running for next year, and kids love instant results.
Brian