I was referring to the machines they were preventing the installation of Firefox on.
For BYOD, I don't know what you're gonna do. Many students have smartphones too (some with tethering), and you can't control what they look at on those either. Plus, even if the school could somehow magically lock everything down 100% within the confines of the school building, the students can still get access to whatever at home, or using coffeeshop WiFi, or whatever.
You can use special paint on the buildings that blocks RF. There are also cell phone jammers. They require a license and approval from the FCC and have legal implications / risks.
This use case wouldn't get approved. It's very hard getting an exception and this doesn't come close to meriting it.
Preventing cell phone calls could have dire consequences in an emergency, and stopping kids from looking at porn doesn't remotely merit taking that risk.
Blocking RF is illegal if it's done with the intent you describe. It's fine if your building gets terrible or no reception but if you purposely design it that way you're not protected.