Or, more likely, the teacher's briefcase.
Yes, using a computer feels a lot like going to an actual physical place. But we forget, it's a portable piece of property. It contains sensitive documents, but it's not breaking and entering.
> but it's not breaking and entering
Well, nobody said it is, including the prosecutors, who are charging the kid with some information-technology-specific crime (informally, "hacking"), and not "burglary".
It's called an analogy. "Breaking & Entering" is the first thing that comes to mind when thinking of a situation where the act of someone gaining access to something is itself a crime. In this case, the student is being charged with "hacking" which according to the Federal law is gaining "unauthorized access" to a computer system.
What would be the crime committed in breaking into the teacher's briefcase? Theft during the duration of time that the student gain 'possession' of the briefcase to open it?