Possession of a firearm on school property does not have substantialimpact on interstate commerce to allow Congress to regulate under the Commerce Clause.
because
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The federal government is granted enumerated powers by the Constitution.
The Supreme Court has provided for 3 types of activities that can be regulated: channels of interstate commerce, instrumentalities of interstate commerce, and those with a substantial impact in interstate commerce.
Possession of a firearm on school property clearly does not fall into the category of channels of interstate commerce or instrumentalities of interstate commerce.
Even accepting the argument that violent crime has an impact on interstate commerce, one would still have to assume that the simple possession of a firearm would lead to violent crime which would impact interstate commerce.