Gun Free School Zone act includes an exemption for people licensed to carry in their state
This is patently incorrect (in the context of this discussion), and will get you arrested and charged with a felony.
The "exemption" for in-state permit holders to the federal GFZA only applies to people who are within the "1000 foot zone", NOT to actually being ON school property or in a school building.
If, for instance you are in Virginia, and you have a VA-issued permit (you don't need to be a resident of the state the school is in--only your PERMIT needs to be issued by the same state) then you can travel within the "1000 foot zone" without violating the law.
Cite:
http://codes.lp.findlaw.com/uscode/18/I/44/92218 USC §922(q)(I)(B)(ii):
(ii) if the individual possessing the firearm is licensed to do
so by the State in which the school zone is located or a
political subdivision of the State, and the law of the State or
political subdivision requires that, before an individual obtains
such a license, the law enforcement authorities of the State or
political subdivision verify that the individual is qualified
under law to receive the license;
But even if you have such a permit, it does NOT allow to carry onto school property unless State law specifically permits it (and even then, you can only have it in your vehicle--you may not step out of the car whole on school property, or enter a building).
A person may also be exempted from the GFSZA if they are traveling under FOPA, and their firearm is unloaded and locked up, per FOPA transport guidelines, even if they do not have a permit, per §§(iii)...
This law, of course, only applies to K-12 schools. Universities and colleges have their own set of laws which are state-specific, and are NOT included under the Federal GFSZ act...
Nowhere in the GFSZA (18 USC §922(q)) is there an exemption for Principles, teachers, instructors, or ANYONE ELSE (except LEOs in the performance of their duties)