Why can’t he be charged?

The Herald reports:

As blood dripped from stab wounds inflicted by one of his students, maths teacher Steve Hose yelled at the rest of his class to get out.

The 53-year-old, the popular head of Te Puke High School’s mathematics department, was about to start class at 11am with his Year 9 students when one of them, a 13-year-old boy, attacked him with a 10cm kitchen knife.

He was stabbed twice in the back of his neck and twice in the shoulder. …

Mr Clement said the boy – who because of his age cannot be charged with a criminal offence – had been co-operative with police who questioned him.

Hopefully one of our many lawyers will help me out, but why can’t he be charged? The Crimes Act in S22 says:

No person shall be convicted of an offence by reason of any act done or omitted by him when of the age of 10 but under the age of 14 years, unless he knew either that the act or omission was wrong or that it was contrary to law.

I am pretty sure he knew that it was illegal and/or wrong to stab your teacher with a knife.