Under normal circumstances, it certainly may be challenging to demonstrate that a school or school district acted negligently in either hiring, or retaining a teacher; there are instances, however, where the playing field may be easier for a plaintiff, as the schools will be compelled to explain their rationale for their action(s)/inaction(s).
"When would that happen?" you ask.
When a teacher has a documented history of sexual abuse of his/her students, and the school either fails to do its due diligence before hiring that teacher and/or fails to monitor properly that teacher on its watch.
And that seems to be precisely what is happening in a case coming out of Southern California, where lawsuits are in the process of being filed for a teacher's alleged sexual abuse of her students over the course of several years.