Untitled Sighting

During the summer of 1987, I was hiking with 8 other teens and 3 adult instructors in the Three Sisters Wilderness in oregon. We were heading up a low ridge around dusk, over to the left and towards the base there was a small pond and as we reached the top there wad a small lake to the right down the other side of the ridge. The instructors set up camp further up the ridge about 50 yrds from us. We were setting up our tarps and collecting water for the night, making dinner ect. The sun was down but it was still light enough to see clearly for maybe 30 more minutes. Alex when to relieve himself, and we could still see the instructors up the hill from us. When al of a sudden a rock about the size of a bowling ball came flying into our camp!! We where shocked, then started yelling at "alex".. "Knock it off!!", ect... then another one came and another.. The rocks were not too close to us but close enough to be somewhat of a danger. Then Alex came back, we all got in his face and were really upset, then another rock came down!!! We all ran up to where the instructors were at, and told them what happened.. They of course thought we were full of it, when from out of know where again came anther rock not so close but again close enough.. It is getting quite dark now and all sat back to back in a circle, with our ice axes in our hands.. I think we stayed up all night, but the next day we all just left and never spoke about it again... weird huh?