Mark Twain: "Never let your schoolin' interfere with your education."
If what is being taught is useless to you, learn something else in that moment. Try figuring out how to teach a particular subject better, or how to test for successful learning. Try determining what the teacher ought to do to control that class clown, or understanding why people talk during class despite repeated warnings. Heck, try to see if you can guess why the teacher has particular things on his desk, or why the room arrangement was done as it was.
From every given situation, there is always more to learn than you could possibly absorb. Maybe you could figure out a supremely discreet way to pass notes during class.
I use AIM.

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"I read, I interpret, I think, I criticize, I oppose, I listen, I write, I question, I reply, I quote, I tell, I name, I discuss, I interpolate..., I learn, I teach, I live, therefore I am." -- Marc-Alain Ouaknin, "Mysteries of the Kabbalah", p383.
Favorite Essays I Wrote:
love,
identity & growth,
economics,
education,
equality,
definitions.
Recent Books I liked:
Anansi Boys,
Fly By Night,
Hyperion.