My good friend broke his collar bone in high school doing the same thing. It was lunch time, and a lot of the guys in our grade were in the 'Senior Study', a big room with computers and desks for students in year 11/12. We found some gym mats in the room, and my friend suggested trying to jump over a lot of chairs.

He set up 7 chairs in a row from the get-go, and after a few practice approaches, he clears all 7 chairs perfectly and lands in a forward roll. At this stage, nearly everyone is like "Holy shit, he just did that", but some people who weren't watching encouraged him to do it again. The second time, he landed badly (although he managed to clear the chairs again), with his shoulder/head hitting the ground first.

He gets up and tries to walk of the pain which he would later discover was his broken collarbone, but it gets so bad that he can barely stand. He realises his collarbone is probably broken (it was pretty obvious - there was a large protrusion where his right clavicle usually was) , and at this stage all but one other person leave so they aren't involved. I get the other friend to look after him while I run to the school office and get them to call an ambulance, but not before the three of us agreed that it would be best for our year and the future of the 'Senior Study' if we said that it happened when he had slipped on the stairs outside and landed badly.

We managed to convince our deputy headmaster that this was in fact what had happened, and my friend was carted away by the paramedics. He now has a nice titanium bracket reinforcing his clavicle that reminds him not to tempt fate.

BONUS FRIEND INJURY STORY: The same friend was recently riding home from work on his bike when a driver opened the door of their parked car without looking, resulting in my friend clipping the door, being thrown off his bike and almost run over. The tangled mess that was once his bike is the only reason the car driving behind him didn't also run him over. He walked away with a fracture L2 vertebra, minor L1 fracture, and a L5/S1 disc protrusion.