Thanks for your questions!
We've designed the game to be enjoyable for adults as well as kids, so there aren't really any overtly chidish elements in it. Apart from the fact that the adult players will be in the roles of teenagers, they should be able to play in a normal adult kind of way. We've playtested the game with adults from 20s up to 50s, and got great feedback, so we're pretty happy that we've struck the right sort of level.
The characters do have abilities – each has a mix of spells and more conventional capabilities. We try to keep 'system' to a minimum (as you say, we don't want it to get too LARPy), but there's enough to make a genuine 'game' element to using magic interactively. Of course, getting good grades in the exams is important, but players may find that other uses for their magic arise during the game that are even more important…
Most importantly, I should say that we offer a no-quibble refund in case of dissatisfaction. So if you buy the game, read through it, and decide that it's not going to be interesting/thoughtful/etc enough for your guests, then we'll refund you in full right away. We've not been taken up on this very often, so we offer it as an indicator of our confidence in the game's enjoyability