As I understand it, he accessed his site with school property. While I don't agree that the punishment matches the "crime", I'm sure there is some stipulation that when he was assigned the laptop, it was for school use only.
How is this different from where you work? I know my company's HR will swoop down in a heartbeat and will discipline anyone using company assets to access non-business sites. And guess what, they don't block those sites, because they believe in accountability and responsibility.
Like I said, I think the punishment is extreme in this case, but what a great lesson to learn so young in life. When given expectations, there are consequences for your actions if you don't abide by those expectations.