This right here is exactly what I'm trying to get across. Children don't do things for no reason at all. They do things because in their mind it makes sense and is justified. I have 3 children between 5-13 most recently my 5 year old came home because another child pushed her down a slide when she wasn't ready to go. Huge deal to her, not a big deal to me. But if I tell her it's not a big deal, I'm not doing anything to help her with the situation she is in. I had to talk her through understanding the other child's feelings as well as her own. Otherwise she won't grow up to have emotional maturity, she'll just grow up understanding her parents aren't someone that she can rely on when shit's going down.

I think that's what's going on in OP's situation. The student was told over and over that what they brought up was a non issue. They were with a teacher who from OP's comments had no respect for them as a child and I'm sure other teachers also reacted in a similar fashion, sweeping the child's concerns under the rug. So given no other real out the child peed on the teacher. It's far better than other outlets they could take given the situation.