Being blamed for somthing i seriously dont think i did!?

Alright so me and my friends were at the pool a couple of weeks ago and we were havin fun filling up some water ballons and things like that ect. And me and one of my friends decided for some reason to start throwing rocks around the pool area( I igmit not the smartest idea) so we wanted to scaare are other friends so we chased them into the mens locker room with the rocks and evry time 1 of them would come close to leaving the restroom we would throw another rock at them. After that me and my friend who was also throwing rocks jumped on our and rode off when out of nowear this crazy lady was running aftr me and my friend and she was like "I know who u are!" and "You better stop!" and I knew i have never seen this lady in my life and she didnt know me so after each thing she would yell at us i would yell "ok!" and "Ya!". I guess she was pissed at that point and we were long gone up the rode but she walked into the pool area and grabbed one of the kids in the locker room and took my friends out of the locker room and asked who they were and what they were doing. Me and my friend on bikes had a feeling they were gonna rat us out so he went back to the pool are while i just went home. One of my wimpier friends told the lady my address and my name. So two weeks later they sent my parents a letter from the board of directors saying that i had broken a big light fixture in the northern part of the pool area. They want me to pay for it and if i did do it its around 400$!! i serously dont think i did it but when I walked up to it, it sorta looks like a rock broke through the plastic peice covering it. So i have no clue what to do and im sorta freaking out because im the only one in trouble and im not ratting my friend out who did it also! so any ecxuses? ideas? anything helps! thanks