No you can sue them in pretty much all cases if they put their hands on your kid.

I think you are dreaming.

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?

There are toll-free hotlines for reporting teachers engaging in child abuse.

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.

Have you any data for how well that works for poor black children?

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