Yes it is. He didn't hurt anyone or even mess with their files in a naughty way. I was so happy to get away from school and this kind of bullshit.
That's exactly the kind of thing the CFAA was created for.
I agree with the GP that he was harmless, and doesn't deserve anything terribly serious as punishment. But what he did is a lot more than using simple HTML injection to add a rick roll to something.
'But he achieved it using leet hacker skills' should not be a factor in determining the nature of the charge or potential sentence. That's along the lines of making a big deal out of someone using a "Subversion" system to access and maintain code.
Getting access to the administrator account is pretty different.
I've had people deface sites. It's stressful, upsetting, and a lot of work. You can't really trust a compromised server, and school IT is fairly unlikely to have great processes for it.
> I was so happy to get away from school and this kind of bullshit.
The real world isn't likely to look any more fondly on this sort of behavior.
While finding the problem should never be punished if it's responsibly disclosed, exploiting it (publicly in this manner) should be.
I also hacked the online grade review site, which was trivially hackable (javascript password protection, wtf), told a teacher, nothing happened to fix it (or to damage me) and I dropped the issue.
Both of those were done in attempt to be altruistic but technically both were completely illegal. Since no administrator ever found out, no cops were ever called and no prosecutorial discretion was in play, but I wouldn't want to bet I'd have been "let off", which is sort of terrifying.
Yes, it probably should have resulted in a suspension, not a felony charge. No, it's not as benign as you implied.