This question already has an answer here:
During GoF in the Harry Potter series, Profesor Moody
(who is actually Barty Crouch Jr. using Polyjuice Potion)
teaches the kids in his class about unforgivable curses, but then goes into it in detail about them and how they are pronounced, how they work and even how to resist some, like the Imperius curse. He even went as far as saying
"Earn you a one-way ticket to Azkaban. Correct. The Ministry says you are too young to see what these curses do. I say different! You need to know what you're up against. You need to be prepared..."
He also has them practice resisting the imperius curse
"The Imperius curse can be fought, and I’ll be teaching you how, but it takes real strength of character, and not everyone’s got it. Better avoid being hit with it…"
It would seem that teaching students spells to protect themselves from the ambush that you're a major contributor in is not the best plan. That's like arming an enemy country with a missile defense system just before your about to attack them with a missile. And i know that some might say that that he didn't think that Harry would be able to defeat the dark lord anyway but still, why not just teach the 4th year Defense Against the Dark Arts textbook information and be over with it?