Throwing a fire extinguisher even into a space where no-one was standing at the time of throwing, seems an incredibly dangerous act.
Without having gone into what was in the fire extinguisher, (ie foam or water) it is clear that you are in effect throwing a small bomb, because the contents are under pressure and the extinguisher has not been designed to withstand dropping from that height.
So yes he was immature, and did not injure or kill someone, but the simple fact was he knew nothing about whether someone might have walked / run into the path of that extinguisher, and when it hit, could have thrown out debris to kill or maim dozens.
It would not be on a par with a nail bomb, perhaps, but in my book comes close.
So he gave himself up, so what. Someone, somewhere, might have (after a spell drinking) bragged that they knew who threw the extinguisher, and it could have leaked that way.
Did he deserve a shorter sentence? Probably not. After all, his was a serious crime and given the likelihood of repeated actions by students, seems likely that someone somewhere was going to go far too far. He was the first, and might not be the last. It serves as a warning (which all penalties do) that there is a penalty to pay for misdeeds such as that.
Motoring offenders getting off with less ? Well sentences should be harsher for them, then, don't try to argue a lesser sentence needed for him simply because someone wasn't killed. While it would not easily have been classed a murder (he could have struck anyone who walked under the extinguisher), it comes down to a crazy act and clearly causing distress and danger to anyone at ground level within 100 metres.