The third, set first, in the series. Enjoy.
Harry was bored, and decided to express this. "I'm bored." Ron merely grunted, where Hermione gave him a scathing glare from the top of her book. He tugged Ron away from staring at the ceiling and explained his master plan.
Ernie and Justin were vital parts of this plan, along with Hannah and Susan. So, they lured them into the Chamber of Secrets, using a slight compulsion charm-very legal, though. And then, they left cannabis, and 'magic' mushrooms down there.
The Hufflepuffs didn't come back up for quite a while.
When they did come up, they all looked slightly ill at the sight of Harry, then walked up to Dumbledore and announced, "Potter lured us into the Chamber of Secrets and gave us drugs."
Ron and Hermione gaped at Harry.
"Drugs? I thought you said they'd just make them see things from an altered perspective?"
"Harry, how could you? This could very well destroy any chance we have of winning the house cup!"
Harry merely grinned and said, "I told you I was bored."