Any State can fix this problem if they so choose, but it would take balls and integrity.
You pass the exact same law at the State level with one minor change:
There shall be NO exemptions, exclusions, privileges or preferences for ANY Federal Employee.
Then you Strictly and Vigorously enforce it against every Federal Employee in the State. If a Federal Employee drives by a School in possession of a firearm, you ARREST AND PROSECUTE.
You pass the exact same law at the State level with one minor change: There shall be NO exemptions, exclusions, privileges or preferences for ANY Federal Employee.
Then you Strictly and Vigorously enforce it against every Federal Employee in the State. If a Federal Employee drives by a School in possession of a firearm, you ARREST AND PROSECUTE.
No state has a big enough army to do what you suggest. It is a direct assault on federal power. The federal courts would likely rule it to be unconstitutional, but maybe not if you only went after off-duty federal officers. Maybe we can build state militias large enough to enforce it, but right now, that is not the case.
If we had the political will to do this, we would already have won.
Gabriel Metcalf's case is the best chance we have of a test case to take down the law in the courts. So far, no one is stepping up to the plate to help him and his mother.