Not that I like the idea but teachers typically also had/have phones at their desk (for the last couple decades at least) it's just easier for the teacher to have the central office deal with contacting the parents and getting the kid with a fever picked up or whatever. Similarly, it's easier for the parents to call the central number and be immediately responded to and directed accordingly than try to find the specific teacher's number and hope it's a good time for them to pick up directly. For 911 level emergencies in class the teacher would call direct as the lesson plan and class conduct is a bit less important than the kid who can't breathe.
The other problems with this solution is there is no way to just jam cell phone signals inside the school only, the signals that need to be jammed are different each place and change over time, and it's unpopular with both anyone that had a legitimate use case for cell service as well as the FCC doesn't like doing it. It's also a generally complicated solution, even if you only attempt to get "some" of it to work right.
The other problems with this solution is there is no way to just jam cell phone signals inside the school only, the signals that need to be jammed are different each place and change over time, and it's unpopular with both anyone that had a legitimate use case for cell service as well as the FCC doesn't like doing it. It's also a generally complicated solution, even if you only attempt to get "some" of it to work right.