In this particular case, the password was widely known. The teacher must have known that it is known, yet didn't bother changing it, thereby tacitly authorizing those who know the password to use that computer. In effect, the computer was public to the same extent like school property, such as a blackboard. I'm sure the defense will wield some version of this argument.

> 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".