If you're lucky, the PBX they use will have some kind of a cordless phone handset option. Then you could just hook up the base of the cordless somewhere and do your speaking over the intercom from a better hiding place.
Personally I don't think they're going to give you much information on the school's PBX, if any. School officials may be dumb, but kids have been getting on school PA systems for as long as they've existed. They've likely witnessed every trick imaginable by students wanting to broadcast on the PA.
You should get the manual for their PBX now and start studying it. Maybe there's a way to override that administrator's code. Or maybe there are other default codes in there for techs or something. Maybe you could think of a way to call the office and SE the code from them.
