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.
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.
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.
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.