Presumably, Gaba if I committed the 'made-up' crime of hacking, and hacked into your bank's servers, you wouldn't report it to the police?
As others have said, the boy committed a crime - and could have had access to highly sensitive information - SEN files, medical records etc - and given what children are taught in schools now, I am pretty damn sure he knew what he was doing was wrong.
If he was genuinely worried about the security of the school's IT system, he could have approached the IT teacher, and shared his concerns, and even offered to demonstrate - that would have been a responsible way to approach the problem.
"I broke in to demonstrate how insecure your security is" is bollocks. If a teenage criminal broke into a house, would we expect the owner to be grateful for the lesson in how insecure his house was, or would we expect him to want the criminal punished? Most would say the latter - though I am sure gaba would shake his hand, thank him for his help, and send him on his way.
I do think that cases like this would suit a system of restorative justice, where the offender has to perform community service (in this case, at the school), and then, if it was a first offence and the offender was under 18, perhaps no mark on their record. This would, I think, reflect the fact that a crime had been committed, and that some form of punishment was required, but that no major damage had been caused, and the offence had been a relatively minor one - so community service and no record could be a proportionate response.
I do think it is very unfair to blame the IT teacher, and to suggest he/she is acting out of hurt feelings - schools have rules, and when those are broken, a report is made, and the Senior Management Team will decide how to proceed. It would have been their decision to involve the police, not the IT teacher's.