We were doing motion under constant acceleration in specialist, and my teacher said something like: when the object reaches its maximum height, gravity changes from negative to positive. Yeah, we all pretty much failed that.
I think it was my first maths class in high school and the teacher said: Now I know you're all probably busy playing with your playboys, but in high school, your studies are important too.
Not so much a favourite (more of a pet peeve) that I remember from Year 12 English: My teacher spelling out the most simple words. 'Alright, the text we're studying is a memoir. M-E-M-O-I-R.' We wasted a lot of time on her wondering about the pronunciation of VCAA, and her laughing at things that we wouldn't be able to get because it was 'too complicated'. She basically treated us like retards, though I guess in her defence, most of us were.