So they shouldn't have done it.
> Okay, let's all be worker robots :)
It's not about what you want to do. It's about what some low-paid worker who has to clean up after you thinks. Or some other student inconvenienced by your prank thinks.
If you're impacting on someone else's life then you're in the wrong!
What if these people don't want your sense of humour imposed on them?
I think it's ethically wrong.