I would imagine a school PA system would be wired, rather than any kind of wireless signal being involved. 'Hacking' a wired PA system would not involve computer/wifi/Bluetooth hacking of any kind, unless that PA system was linked up to a PC somewhere. Most probably, it will be an old-school microphone-linked-to-soundboard-linked-to-speakers system, which is not 'hackable' in the general sense of the word.