By messing around with potions, do you mean drug use, or his, uh, innovative ways to do Potions assignments? ... actually, it's probably about the same - a lot of people know he has issues with this stuff. Gamzee doesn't try to hide the fact that he smokes and drinks when he goes home, and he's offered people drugs before. He's decent at keeping it under the radar from the teachers, but it's something that at least members of his House and year would probably be aware of. Likewise, House and year would all probably know he screws around with potions, and is weirdly good at it.

tl;dr version: Wendy can know any of that if you want. XD