I think the teacher may have been slightly out-of-line telling the kid to use the lunchbox. I know she was just being a smart-ass, and didn't mean he should literally piss in the lunchbox, but some kids aren't exactly the brightest crayons on the box... and will sometimes do exactly as grown-ups say, regardless of how inane the request may be. Some kids just don't have a whole lot of sense, and then there are others who deliberately do dumb things to get a rise out of their classmates. I think both of these scenarios apply to the kid in question here.

Still, as anyone who actually has kids knows, when they gotta pee, they gotta pee! The teacher should have asked the kid to wait until she was finished with her lesson, or something along those lines, instead of reacting the way she did.