I'd explain my kid my perception of the situation...

School is not made to learn stuff, it's to get a diploma to give you access to better job in the future.

It's important to know the truth, but don't lose track of your objectives. Learn to play the game and get good grade. When you have your diploma in hand, then fell free to tell your teacher what a moron he really is.

I'm more a Peter Keating than a Howard Roark