Just turn on your voice recorder and go.

I can't imagine any good reason for that policy other than covering for terrible people.

Is there an expectation of privacy in a public school classroom?

Have you done that? Sue the powerful (a school board) from a position of powerlessness (black working class in the ghetto)? Probably not, unfair question. Do you really think it is a realistic prospect? Really?

Obstructing a child from using the bathroom who needs to is a form of abuse. If that teacher then physically restrains or otherwise contacts/touches the child to prevent them, especially if it culminates in their wetting/soiling themselves, this starts resembling something along the lines of sexual abuse.

If there's one thing the USA can be relied on it's both society and the law leaning heavily in the direction of protecting the children, to a fault.

This situation is literally just a phone call away from completely ruining the teacher's career if not life.

I hear people with theories, but child services the world over are famous for nice theories and systematic abuse. Especially for minority and poor children

And being the teen that I was, it's not like I had an otherwise perfect attendance in any of my high school years.