My Maths teacher (she's the greatest!) was encouraging all of us in 2 Unit Maths to buy a Mathaid so we can sketch graphs. She was really convincing us to get it, realised how she sounded and said, "And you also get a free set of steak knives!"
=] Ah, Year 11 Maths...
EDIT: I've thought of more!
My Maths teacher (same as above) was still getting used to the Smartboard in our classroom. She was using both the whiteboard and Smartboard and accidentally wrote on the Smartboard with a whiteboard marker. We all noticed and cracked up laughing. She quickly erased the mark on the board and chucked the whiteboard marker out the door so she wouldn't do it again.
WAYYY back in Year 7, we had a new teacher for Science. It was his first year at our school, and being the little shits we were, we picked on him about everything (he's actually a really awesome teacher and the nicest person). His only whiteboard marker ran out, so he went through the bin in our classroom, picked out another whiteboard marker and used it the rest of the lesson.
My Chemistry teacher was talking about the different structures of carbon molecules. Being a Thursday, we were all tired after sport and were practically asleep in the classroom. He started talking about the "condom" shaped carbon molecule and, surprise surprise, we all paid attention. He said that that's what condoms were named after, afterwards we all went back to sleep.