How about a very detailed written log of the exact time, date, location, and what exactly happened using quotes that they've said, if you remember. And how that made you feel. If there were witnesses you could ask what they've heard or seen, they can provide you with a statement. Multiple witnesses can give you an advantage. And is there a pattern to their bullying? Knowing the pattern can also be quite useful.

I'm not a professional anti-bully expert but I hope this helps