Actually, IIRC, the original form of the GFSZ law was struck down. Congress repassed it, adding language stating why they think the Interstate Commerce Clause justifies their authority to pass it.
If you carry a gun in a school, expect to be arrested. LEOs will find some State law to use. If the State does not have a GFSZ law, they will simply use DC. So, they don't need the federal law to arrest you. However, if you are in violation of the federal law, you can expect to be also (or only) charged under it.
I don't know of any charges ever filed using this law from casual carry within the 1000 feet. I suspect that, if any officer is foolish to try to arrest someone for carrying withing a 1000 feet of a school, the SCOTUS will, once again, strike the law down. It is still on the books because no one has been foolishly charged under it--yet.