Is there any chance of a transmitter that would transmit through my school p.a system?

Question

its a practical joke by the graduation class

Answer 1

Not really.  At least not without a receiver for the transmitter connected to the p.a. system.

Answer 2

any PA installer worth his salt has shielded the system to block RF interference.

Answer 3

Any wireless mic would work with your school PA system.  You will need access to the mixer.  If you get permission to access the mixer,  you would just need to connect the microphone receiver to an unused channel on the mixer.  The microphone (transmitter) will need to stay pretty close to the receiver at the mixer though.