> the students can still get access to whatever at home, or using coffeeshop WiFi, or whatever.

That's fine, these are not school responsibility. Once the parents complain, you can redirect them to their home or coffeeshop.

What stops them from downloading this stuff and still bringing it to school?

The point is not stopping them downloading this stuff.

The point is stopping them downloading this stuff using schools property or infrastructure. If they download it elsewhere, it is someone others problem then. If someone complains, its someone elses' fault, and the school can fingerpoint.

If they just bring it to the school, they can be disciplined, but no other steps need to be taken.

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.

I completely agree.

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.

There are a number of organizations and businesses that block RF. Their legal team review the local statutes and the employees sign acceptance in the AUP onboarding documentation.

To your point, there are certainly countries and jurisdictions that do not permit blocking RF or have strict exceptions.

> Their legal team review the local statutes

In the US, local statutes and AUPs don't enter into it. This is federal law. You're right, there have been a number of organizations that have done this -- and enough of them have been fined for it that the number is much smaller than it used to be.