If my child regularly sustained serious injuries at home, Child Protective Services would eventually stop accepting my failure to supervise her as an excuse. Schools should not be allowed to get away with it.

This problem can be solved. The trouble is that it probably requires money, which taxpayers really hate to provide.

Surveillance cameras are pretty cheap these days.

That's going to change the very second a wearable emergency 360 deg camera starts to be marketed to parents of bullied kids, and is adopted widely.

As Mikeash said, it's a he-said she-said situation when there was no supervision. If it is frequently (or even usually) a he-said she-said that means there is not enough supervision.