THE CURE:
https://www.walmart.com/ip/5pc-5x7-Cellphone-ESD-EMP-7-0mil-Faraday-Bags/472221450
Drop your device into one of these and “go dark.”
You can do the same thing with some aluminum foil.
If you are going to use one, make sure you turn off your phone before you put it in the bag or put your phone on Airplane mode or your phone will quickly drain your battery trying to find a cell tower.
Of course using a Faraday bag defeats half the purpose of having a cell phone; being able to take calls.
The school will call the parents and tell them their kid has probably left the campus and they don't know where he/her is.