Of course the right to privacy applies to children as well, and to messaging in school between peers, just like in civilized jurisdictions it's illegal for employers to read read email between employees.
Except schools have a responsibility to ensure that they safeguard their students against online abuse which is extremely common between students. Students will routinely bully each other over school messaging systems so it's important schools have systems in place to deal with this.
This sounds like just an excuse for spying on students. There is plethora of methods to prevent bullying that do not require an all time access to students' communication.
Pervasive monitoring of private communications doesn't sound like the right tradeoff - more like "something must be done, this is something, therefore we must do it"[1].