Replying to I need your help Catching a scumbag

So there is this kid in my gym class that stole my phone in the locker room a month ago. I talk to my dean and friends and there was nothing they could do due to the fact that there was no evidence as to who had taken it. I had a good idea who might have stole it and most people I know agree with me. Today my friend had his phone stolen in the locker room during wrestling and again, the same kid is suspected but we can't do anything because we have no evidence. Do you guys have any idea as to how we could get evidence to prove it was this kid? Thanks guys