There has been cases of 17-year-old minors raping and killing that went completely free (or spent a few months in juvie).
What I'm saying with this is that yes, I agree with you: The age of the perp should be considered. But we should also consider the crime in question.
In this case, both age and 'crime' account for what is just a prank with no harm done (apart from bruising some teacher's ego). Even expelling the child would seem to me a bit excessive, but I could understand the school doing that. Anything more is just disproportionate.