Leaving aside the moral question, I feel compelled to ask "Why was anyone surprised?". Hollywood has glorified computer thievery and hacking for years, with films such as "War Games" showing EXACTLY this offense, and "Swordfish" portraying a computer expert receiving sexual favors while breaking into a government site.
As if that weren't bad enough, other movies have shown computer invasions and misuse by all manner of "good guy" characters, inferring that the end justifies the means, even if the "good guys" were doing so without proper supervision, without accountability, and without penalty.
Small wonder, then, that children feel it's OK to break the rules of civilized behavior so long as a computer is involved. To compound the felony -- pun intended -- school departments and government agencies at many levels treat computers as electronic typewriters that are "safe" so long as they're located in municipal buildings, without regard to the larger question of how such systems came to be used for grading without any serious effort to conduct a security audit, or to educate their users, or even to question whether the computer system involved should have been connected to the net at all.
The criminal charges being thrown at this young man are, of course, an over-reaction, attributable to the embarrassment he has caused those in charge of the system, and especially to those in charge of those in charge.
IMNSHO, this was an incident waiting to happen: an attractive nuisance no different than a stepladder left leaning against the side of a building where children could use it and thereby be injured. We don't blame children for climbing ladders: it's what children do.
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A little learning is a dang'rous thing; Drink deep, or taste not the Pierian spring; There shallow draughts intoxicate the brain, And drinking largely sobers us again. -- Alexander Pope, Essay on Criticism