There is one very simple prophylactic against such abuse: set out your ground rules clearly in advance. There are two ways of doing this and the links here will take you to mine, as examples:
- Have a website page on which you explain what sort of things you do (and don't do) and what sort of recompense you expect.
- Have an event terms and conditions document which you can send to any school which contacts you. (Or which can be downloaded from your website.)