Honestly man, leave it. Only because you'll never be found out. Not like they'll get the police involved. :dumb:
I've seen something like this happen to my mates, all the school did was ask, and make school announcement's about it. He never got found.
If the day does come to were you do get found, down right lie. But sure, say, "He bullies me, so I wanted to make him look stupid by putting that in his drink etc..". To be honest, it wasn't your fault. He tripped because he was the one not paying attention about were he was running.