This appears to be a regular fault with that range of Canon camcorders - which means that you haven't broken it yourself (probably!).
Doing a quick search on here it was mentioned nearly 2 years ago
Remove Cassette Message and comes up a handful of times in the mammoth
MV600 Problem although that's mainly about the 'Black Screen of Death' image sensor failure problems. But I can't see whether Canon have actually recognised it as a design fault and therefore whether they've ever offered to repair it free of charge.
Depends on the age of the camcorder (and what sort of use it's had!), but worth the teacher contacting the original retailer, then possibly Canon, then possibly Consumer Direct (Trading Standards). Sorry can't help any more than that.