Then some kid has a medical emergency, people take 5m extra to call an ambulance due to having to go outside or find a landline, and the school gets sued.

Get in line? The schools (in CO at least) are getting sued all the time. Most of them are frivolous (I want my kid to play on varsity, the school lunch smells bad so I need to bring my dog to class, I have ADHD but am allergic to plastic(?) so give me an A) . Some are legitimate and mostly about classroom sizes and racism/sexism. Some kids bring guns and knives to school, attempt/commit murder and then snapchat themselves doing it. Most of the district's caseload is made of 'slam-dunk' cases, but they do add up and funding for the legal department here is not going up. Classes are now about 37 students/room. They aren't teachers, they are wardens.

Faraday Cages may not be a bad idea though. Copper is fairly cheap. It's making new windows and certifying that the door to the classroom is closed and that no signals can get in. Heck, with the way battery life is going, maybe just take away electrical outlets and power-strips. Only 1st period would be effected with a bit of kids after lunch.