I'm sorry. Four years to decide "Hey I'm gonna throw water balloons at people" is pretty sad. Plus since it's in the middle of the year, getting suspended will jeopardize your chances of passing if
you don't get expelled first. It's considered assault.
Plus there's the fact that water damage is serious business. A lot more serious than people think it is. Just hope none of those teachers bring in laptops, or wear hearing aids or anything they need to get through their days. Important documents are also highly vulnerable.
I had my experience with water damage in a school that I'd like to share with you: http://yart.deviantart.com/journal/25767344/
What that journal doesn't go into detail about is the hundreds of thousands of dollars worth of equipment that was destroyed. Remember: Anything you ruin, you pay for.
You don't strike me as very clever. In fact, I'd say you're more of a straight out idiot to even consider this idea. Perhaps you should take ten more years to think this over.