Yes, God forbid the teachers/students be able to call 911 when nut case kid opens up with dad's guns. Or let LE know where the shooter is in the building- this actually happened recently and the cops commented on how helpful it was to them.
We hire ADULTS to run classrooms as teachers. The phone is one usefull tool my wife has in her class. With it she can contact parents when needed to discuss situations involving the child. She can have the student call the parent when needed - great tool when the kid is lying to her about something.
If a you are aware of a teacher spending her day talking to someone about a sick cat instead of teaching, please contact the teachers principal.
Personally, I'd like my wife (a 6th grade teacher) to have a school phone, her own cell phone, a gun, and a local administration and police department who will enforce the law and back her up when some spoiled brat threatens to hurt her.
It is the school systems phones and computer network so yes, they can pretty much do what they want with the system. They can ban personal email and such. There are some rules however on reading personal emails. Generally they are supposed to stop as soon as they realize they are personal. They can discipline the user, but they are not supposed to read any further. It is still a slippery area.