What’s a polite way to ask the user to verify that they’re not abusing my system?

My app is a game for kids in classrooms, and we offer a demo mode for teachers, so they can try before they buy. We've found that lots of kids are signing up for a demo so they can play the game for free, and then they abandon it when the demo is up. We have people who go through these demo registrations, and it's a big waste of their time, and messes up our metrics on the value of having the demo mode in the first place.

We would like to prevent this by asking the teacher to enter an email address or phone number, so that, before they can get the demo, we message them a short code, and then require them to enter that code. The idea is that a legitimate adult user would have no problem going through this step, but a kid who's just playing around will think twice before identifying himself.

So, what is a polite way to ask for this information and explain why we want it, without sounding off-putting, like "we don't trust you"?

(Alternatively, is there a better way to solve this problem?)