I never said anything about all 50 states. I said federal law allowed an exemption "if the state law on concealed carry allows it". That allows a more lenient state law to supersede the provisions of a strict federal law, contradicting the statement you had made earlier that the more stringent federal law would always supersede. For instance, the Ohio concealed carry law allows possession within a school zone for the purpose of picking up and dropping off kids. Some other states allow more than that. Some allow less. The only thing I may have left out is the background check provision required for the state law to supersede the federal law. But I never said it was allowed in all 50 states, nor did you ask about all 50 states. The federal law does however allow all 50 states, if they require background checks for their licenses, to enact more lenient restrictions on school zone carry.
This actually causes complaints for states that allow carry without a license or permit. Those people actually end up having more restrictions on school zone carry than people with licenses in many other states. There are all manner of technical catch 22s out there that have unwittingly been created by such laws too. If someone wanted to get extremely nit-picky about it, you could be prosecuted for driving on an interstate through a state that has no such school zone exemptions and there is a school with 1,000 feet of that interstate, whether you know the school is there or not. You would have just carried a concealed firearm through a school zone in violation of federal, and possibly state, law. If government can't screw something up, nobody can.