Technology

In the US this isn't legal. Signal jammers are outlawed period, you can't even use one in your own private residence. The FCC is super clear on it's legality. Any form of signal jamming period that can disrupt a communications signal such as a cellphone or wifi signal on US soil is a felony. They have even gone as far as fining companies who sold devices that could potentially have been used to signal jam over the restriction. In the US the only time you can use any device that has the capability to block signals is by a federal agency on a case by case basis

They actually ~~have~~ had (its been replaced with a much shorter excerpt that only mentions classrooms in a passerby) a FAQ about signal jammers where they explicitly mention it's use in the school environment.

I am a principal or school teacher and would like to use a jammer to limit cell
phone calls and texting during school hours. May I do so?

No. It is a violation of federal law to operate a jamming device within the U.S. Cell phone
jammers do not distinguish between social or other cell phone conversations and an emergency call to a family member or a 9-1-1 emergency responder. In an emergency,
use of a cell jammer can pose a threat to both students and staff by preventing
communications with police or emergency services.
In other words, jammers, even if carefully targeted, create risks of interference outside
their intended zone of operations and can thereby disrupt critical communications by
public safety providers, as well as the legitimate communications of passersby.