I think that, as teens, we have all done some rash things without thinking, and right afterwards thought "F*** me that was close" with wide open eyes
He was lucky, and so were the people below
Contrast his sentence to some of the death by reckless or dangerous driving or caused by drunk drivers in stolen cars with no insurance etc ... which are frequently a lot less, but with a dreadful a consequence.
Personally I think it was a show trial with an extreme sentence way beyond what was necessary. He does appear to have genuinely regretted the event and realises it was wrong, and is not a persistent bad person. However, now he is locked up with the rest of the chavs, thieves, muggers and rapists, there is a good chance he will be when he gets out