This story was going around in the early 90s. It probably never happened as each time there was a different ending to the story.
The teacher was late for lab and when he was heard coming some guys turned on the gas and they all slumped over (either their desk or the floor) with their tongues hanging out the side of their mouths.
Here are the different endings I heard:
1. The teacher got up on his desk and danced a jig,
2. The teacher grabbed a chair and smashed a window,
3. The teacher went home and called in sick,
4. The teacher came in, gave his lecture as usual and left.
Feel free to make up your own ending.