The question was asked in this thread
in which Supermoderator Mike briefly participated. Several comments suggest that 18 USC 922(q) has only been used as an enhancement to other charges. These comments have not been contradicted since.
I found this
SCOTUS No. 93-1260. Argued November 8, 1994 Decided April 26, 1995
After respondent, then a 12th-grade student, carried a concealed handgun into his high school, he was charged with violating the Gun-Free School Zones Act of 1990, which forbids "any individual knowingly to possess a firearm at a place that [he] knows . . . is a school zone," 18 U.S.C. 922(q)(1)(A). The District Court denied his motion to dismiss the indictment, concluding that 922(q) is a constitutional exercise of Congress' power to regulate activities in and affecting commerce. In reversing, the Court of Appeals held that, in light of what it characterized as insufficient congressional findings and legislative history, 922(q) is invalid as beyond Congress' power under the Commerce Clause.