After the ruling, Attorney General Janet Reno recommended that the Gun-Free School Zones Act be amended to apply only to any gun “that has moved in or that otherwise affects interstate or foreign commerce.” [sic]Gun-Free School Zones Act United States [1990]
Those 12 words in the 1995 amendment included (read "buried") under Title IV, section 657 of the Omnibus Consolidated Appropriations Act of 1997:
H.R.3610 - Omnibus Consolidated Appropriations Act, 1997
(Sec. 657) Amends the Federal criminal code to limit to firearms that have moved in or otherwise affect interstate or foreign commerce the existing prohibition against the possession or discharge of firearms in a school zone.IOW, the '12 words' were intended by Bill Clinton/Janet Reno et al to overcome the principal shortcoming of the law, that GFSZA is unlawful on the basis of a federal law not governing commerce.
To the best of my knowledge, that amendment has never been challenged.
The law, with the amended language has been challenged several times (at least nine times).
In six circuits, the law was upheld as constitutional, one of those (First Circuit, 2014) later reversed in another case, leaving five circuits which held the amendment to be sufficient.
In three circuits, the 12 words were held to be insufficient to heal the constitutional defect.