Yeah, it would most likely be over a special line in the phone, it seems...I haven't seen those separate PA mikes in the principal's office in real life, ever; it's usually a line on the phone. If you can find a way for that...

I googled "senior prank ideas" to see if anything worthwhile showed up. This is all copy paste, so advance apologies if anything stupid appears -

- Spray painted handicap symbols on all the teacher and administration parking
- Saran-wrap cars {Actually...can't that hurt the cars? O_o}
- Put the school building up for sale in a real-estate ad
- Let out thousands of bouncy balls on the floor {We actually did that before each winter show for theatre...It can get pretty fun}
- If your school has a pool - dye it purple
- There's got to be some way into your school at night. Get in, take all of the desks and chairs, get someone with a ladder and put them on the roof. {Considered this one too}
- They put saran wrap in the toilets in such a way that it wasn't on the seat, but it was above the water... no one in administration found out till someone noticed the smell... it was funny tho...
- They left all the windows open after school in the middle of Jan last year, and it just happened that there was a blizzard that night, so the next morning, the classrooms were full of snow...
- A bunch of the seniors got a school letterhead and sent a letter home with everyone a couple of weeks before prom that said that the school wanted to practice safe sex, so they were going to have free condoms at the prom.
- on another DUMB PRANK note, the seniors last yr did "locker mayhem" aka putting these plastic things on everyone's lockers so we couldn't open them...but they also did THEIR OWN lockers aka pranked themselves.
- Another class removed all board markers from the classrooms, but a janitor got fired for that one.
- At my school last year the seniors started a HUGE food fight in the cafeteria. I mean HUGE...the type you see in movies. {This happened at my school because my friends got bored x_x}
- putting icey hot on all of the toilets

Another idea one of my buddies had when I was a senior was to take some plywood walls and install them in front of the main entrance some dark, Sunday night. We worked it all out - how big they'd have to be, how to stand them up, how to connect them to the walls so it's not necessarily easy to take them down...He got lazy though, so it never happened

Any of those tickling your fancy?