Thank you for sharing the form.
Do you know which field is the one that gives the error? and which browser the user used to fill out the form?
Is this problem happening recently?
One thing to try on the form filler side is to clear the browser cache.
There seem to be two places where the users are having problems - the question that asks about transportation - I'll often get a form that has number of buses and that they are walking, for example. Also, the form will sometimes require them to choose all three concerts. I set it up so that they could choose to attend one, two or all three.
I don't know which browser the teachers are using...it could really vary. We have about 200 different schools that fill out the form so there's no way of me knowing what each teacher is using. Should I specify a certain browser? It has started happening somewhat recently.
I don't know if this makes a difference but I took the form I created from scratch last year, and just updated it with this year's information. Last year I didn't have any problems whatsoever. This year, I'm having problems. Does it work better if I create the form new every year?