I love the solution, although I'm a little surprised (esp. in this culture of being offended by everything) that the principal / school didn't get into trouble. I could picture one of the girl's parents suing the school for harassment and / or putting their "little angel" in danger health-wise.
It wouldn't do any good to sue the school, or the principal. After the girls saw the toilet water being used to clean the mirror, if they continued to kiss the mirror, it's on them.
Most of the parents I know would say the principal taught the girls a good lesson.
Except that the principal's story was that that was the current method being used to clean the mirrors... The principle had brought in the custodian to demonstrate how difficult it was to clean, not to demonstrate a new cleaning method that the custodial staff would start using. Then again this would also require the court to assume that the principal's actions and story were literal truth and not just a stunt to get the girls from stopping their behavior. And I too would hope that the parents would also understand the intent of the "demonstration" was to teach the girls a lesson, and would side with the principal instead of their daughters. Unfortunately their are some parents that think their "little angel" could do no wrong, and so will side on their child's side over the school's...
I think we're forgetting that they are children, not just short adults.
Children will, and have always done, things that as adults, we know to be the wrong thing.
Harsh punishments, or even any punishment at all, is not always the answer.
Sometimes all it takes is a little gross-out to change a childs' path.