Sadly, criminal liability taken the tool of discretion away from educators these days. Everything is "zero tolerance" and the educator that finds this stuff and fails to notify the authorities about "child pornography knowingly exchanged in the classroom" is putting their career and possibly their freedom on the line.
It has become easier to just call the cops and "CYA."
Of course you are right. And in my hypothetical "parental conference" behind closed doors, the school would be held liable when little Suzy's father pops Jodie's Dad in the mouth. Why? Because the school district has the "deep pockets", and no other reason.