I was in the exact same situation last year... I had explained that I had used them as revision material in preparation for the exams which coincidentally happened to be exactly the same... I just told them that they were relatively easy to access on a revision website and that's how I came across them... they asked me to show them the website I found them from and I did, when I got the actual exam papers back of course I received pretty much full marks and for 2 out of 3 papers I sat they didn't take those into account towards my final grade because they knew of the circumstances. by the way, I didn't know they were going to use last year's paper in my case, but before I finally spoke to my teacher pretty much everyone in my year knew about it and it was terrifying because half would accuse of cheating and the others pretty much would have revised the same way I did.. but don't stress too much about it because it's not as bad as it looks