I don't think Blockland is built with site licences in mind, but I'm sure an interested school could purchase them in bulk at some discount rate. In fact, they'd only need one key if they used the LAN function built in.
If you think about, $20 is an incredibly cheap cost for a piece of software that could be used across the entire school network. You may as well stress this important fact and talk to the principal of the school about it.