Based on a presumption of liberty, in the state of Missouri, such a law would be unconstitutional.
Missouri Bill of Rights
Article I, Section 23
"That the right of every citizen to keep and bear arms in defense of his home, person and property, or when lawfully summoned in aid of the civil power, shall not be questioned; but this shall not justify the wearing of concealed weapons."
The only qualifier is you have to "bear" your arm in the open.
Bill of Rights and Constitutions enumerating rights primary and sole jurisdiction is on "public property."
Schools that are funded by tax money are inherently then considered "public property."