Depends on the meaning of "illegal"...
At every PA firearms bazaar I've been in attendance at, there is a functionary at the door who will ask you if you have any weapons about your person. If you are carrying your weapon(s) in a concealed manner, you will have a choice before you:
1. Tell the truth, remove and clear your weapon(s), and have a plastic zip tie inserted into the weapon(s) in such a manner as to render them inoperative.
2. Lie, and not disclose the otherwise legal presence of your concealed firearm(s).
Should you choose #2, you can strengthen your appearance of innocence by offering to allow the functionary to search your backpack, which is of course empty in anticipation of the many goodies that you expect to purchase at this particular show.
The zit staff (read: teenagers) who usually serve in these functionary posts have, historically, never challenged anyone who replied to their query in the negative, but MAY have been telling a falsehood...
Disclaimer: I am not advocating, nor will I admit to, my experience with the scenarios as set forth above.
The following represents the Tall Man's opinion only:
When on private property, carrying a weapon when you are otherwise prohibited to do so on said property would be just that: prohibited, to the extent that you can be asked to leave over non-compliance with this policy. The illegal action, in this instance, should be one's failure to vacate (now: trespassing), and not purely because you are carrying a concealed weapon. To think otherwise would be to imply that the property owner's decision to prohibit concealed carry of a firearm has the force of law.
So what about concealed carry on school grounds, Tall Man? Well, FWIW, this would appear to be an issue of 'public' private property, with the requisite legal intricacies contained therein. I believe that properly trained adult staff members should be allowed to carry weapons in a concealed manner on school property. Justification for this logic shouldn't be necessary.
Tall Man