h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

First off you dont have to get caught. Simply you need several things:

1. Distraction
2. Alabis (and people to support them)
3. Planning
4. misdirection

1. Distraction
Have a seperate group of students pull an entirely different prank. Dont let them know whats really going on. plan for them to get caught, or for their prank to be discovered as your doing yours.
2. Alabis
Make sure you have a good Alabi. Also have other people ready to support that alabi. Tell even people you trust the same lie beforehand, just in case they're questioned. Make sure everyones story matches, or that everyones story is so different that they dont know whos lieing and whos not.
3. Planning
Planning is very important. Lots of time should go into planning. You should have lots of lookouts while your doing this. Know your escape routes. Have a backup plan.
4. Misdirection
You want the school to be oblivious to whats going on. Find a kid that you know will report what you plan. give him all false info without him knowing its not true. Do it more than once to make sure that the school will ignore anything reported to them.

Finally make sure that no one gets hurt and that it wont cost lots of money to fix or clean up. As long as the school dosent have to do much they will mostly overlook it. Make sure that there is nothing tying you to the prank.