Yep, I can see where that would be frustrating as a teacher.
FWIW, I am a teacher (not music) and this isn't an uncommon phenomena regardless of subject.
It's the teachers that figure out how leverage their students natural curiosity and access to information that will come out ahead (as will their students). The good news is the student is compelled to learn and seeking more information, the bad news is they don't necessarily have the background to be able to discern what's good information from bad - and that may well be something YOU have to teach!
I guess my point is, to be blunt, get used to it, embrace it and work with it. Fighting it won't work. Get used to being questioned and challenged by students. I'd rather be challenged by a student then have then not have them care one way or the other. It gives you the opportunity to open up a dialogue about why something is the way it is.