Yep, that's adult life.
For a fun series as this, it's not afraid to at least brush on these shitty topics. Like with the sister ending up in a wheel chair. There aren't easy solutions and I prefer it that way. Suspension of disbelief and all that. For example, I'm already getting thrown off by the principal bitching like this. A teacher's responsibilities do not extend to cleaning up the parent's mess. That scene should never have happened. That's not a teacher's job description and it would be incredibly unprofessional. What if a child had an allergy?
There are certainly people as stupid and entitled as this woman beyond a doubt, but the principal and teacher should know better. They should have immediately stated the school's limitations and available options and left it at that.