Umm ewwwwww.
Do you have a garbage can in the bathroom? I cannot for the life of me figure out why they would leave it under the sink. I mean, even if you had no garbage can in there then wouldn't they stick it in a pocket or purse to throw out somewhere? ewwwww.

Why are so many parents using your bathroom? Are you a really far drive from their houses/their work places?

Perhaps it is time to send home a letter noting that with the numerous times you have had to call management to fix the plugged toilets and the findings from under your sink you will now need to refuse use of the bathroom for parents? It's time they start using their own bathrooms and holding it to wait till they are at work!