If you are Hermione in the stands, you have to use the reparo charm on the piles of lumber and then wingardium leviosa to lift the blockade into place. It blocks the bludgers so you can jump across. If you are in the scene after the stands, you have to hit the trunk to release the bludger. The bludger crashes into the platform to release it, nd then you can run up the platform onto the next floor.