This law was struck down by SCOTUS and was immediately re-instated with new legislation by Congress. It has not since been challenged again.
This should be a slam dunk considering the Clinton era Congress showed quite the contempt for SCOTUS over this.
It has been, and been upheld. Think the federal GFSZ law is unenforced? Think Again
Some of the cases aren't on point, but at least one of them is.
Found two specific cites that do contain challenges to the post-Lopez GFSZA, 18 USC 922(q):
US v Danks, 221 F3d 1037 (8th Cir. 1999), where a federal grand jury charged Danks with possessing a firearm within 1,000 feet of a school, in violation of 18 U.S.C. 922(q)(2)(A); and the 8th Circuit upheld the charge against a challenge that the statute was unconstitutional.
There is also US v Tait, 202 F.3d 1320 (11th Cir. 2000) (indictment dismissed on grounds other than defect in federal jurisdiction).