man...you guys make me sad...really, no ideas?

*sigh* nope. You could always look up stuff on google.

I'm a senior too and we did two different things.
1) we took all the tables and benches in the courtyard and hung them upside down from the windows surrounding the courtyard.
2) we [are going to] but cows on the third floor because they can walk up stairs, but not down ;]

i heard that some one let 2 pigs loose in a school and labled them 1 and 3 so when both pigs were caught the search continued for the imaginary pig labled 2

hehe sounds good
i was thinking maybe i could release laughing gas in the school vents
fill all the hallways with plastic cups full of water so no one could get around unless they were wiling to get wet
put clear jello in the toilets
leave mannequins in all the bathroom stalls or place them all over the hallways
saran wrap door ways
take out all the desks in the school
have all of seniors with a stack load of papers with 2010 and during passing time shoot them into the air
wire up the PA to play random techno music every 7 minutes for 30 sec
there's a lot of stuff but i can't decide
helpppp

I'm not in year 12 yet, but here are some things people have done at my high school:

-put confetti in everyones lockers
- take a cow up to the top of the building (cows can walk upstairs but not down - our school had to get a fork-lift to get it down)
-change all the clocks to say your year level (12:10)
-unscrew the desks so when people put books on them they colapse
-Smear lubricant or vaseline on door knobs so they will be too slippery to open
-superglue coins on the ground and watch people try to pick them up

just whatever you do, dont do anything that will get the school too annoyed, or muck up day (as we call it in Australia) will be banned. my school had security guards and everything come the end of the year cos some year 12 class just took it too far.

You could release helium into the office so all the ppl's voices get all high and squeaky...

I was going to say the pig one but someone took it.

If your HS is multiple floors you could have all the toilets in the top floor flush at the same time and if the building is older it would flood the bathrooms downstairs...

There's a lot of others you could do...but I can't think right now, sorry!

I'm not a senior, but I have an idea:
You can make the gym a beach. Get some fake sand, a plastic indoor pool, and backround. Get a grill, some parisols, towels, and a radio. Go with your swimsuits or summer clothes. Hang out in the gym, but make sure to get there early so you can set up before school starts.

Ok...this was not my senior class's prank but still really funny. One time when I was in high school everyone got pulled out of class early in the morning for an assembly and about halfway through this guy ran across the stage wearing nothing but a long white wig like you get at a halloween store. They sent the school cop after him but it certainly make things a lot less boring!

Don't know if this is too late, but one word: Paintballs.