Here is the statute on school carry:
Quote:
§ 912. Possession of weapon on school property.
(a) Definition.--Notwithstanding the definition of "weapon" in section 907 (relating to possessing instruments of crime), "weapon" for purposes of this section shall include but not be limited to any knife, cutting instrument, cutting tool, nun-chuck stick, firearm, shotgun, rifle and any other tool, instrument or implement capable of inflicting serious bodily injury.
(b) Offense defined.--A person commits a misdemeanor of the first degree if he possesses a weapon in the buildings of, on the grounds of, or in any conveyance providing transportation to or from any elementary or secondary publicly-funded educational institution, any elementary or secondary private school licensed by the Department of Education or any elementary or secondary parochial school.
(c) Defense.--It shall be a defense that the weapon is possessed and used in conjunction with a lawful supervised school activity or course or is possessed for other lawful purpose.
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There is no exemption for police officers, active or otherwise.
But police do carry on school grounds presumably because of paragraph (c)
So far we have no case law affirming the defense of "lawful purpose".
People have been convicted of 913 if they were bad guys. And we know of no convictions under 913 of good guys, however, of the cases we've reviewed they were acquitted for reasons other than the "lawful purpose" defense.
If you do a search you'll find lots more commentary on the subject.