The Rock?

I was stuck on the very same thing for about three days.There's a small rock around, you need to wingardiam leviosa it, and put it next to the rock on the Left (left if you've just came from the hogwarts buildings, right if you've come from Hagrid's hut). Once you put it next to the large stone, get ontop of it, and it should trigger a scene.

Reply and tell me if that worked, since it's been a while since I completed the game.

Yes, this is the most frustrating part of the game. No one can really tell you more than what rock to put it by, so you will just need to keep shifting it around until you can climb up onto it and get the cutscene. I CAN tell you, however, that it will be on the inside of the Stone Circle. That's all. I hope it helped!