He commended the teacher for putting himself in harm's way to protect students.
"In most situations like this we ask that people don't put themselves at risk and instead contact authorities by ringing 111, but in this case it appears the appropriate course of action was taken.''
The teacher was not hurt in the incident and the 14-year-old victim did not need hospital treatment.
The alleged offender was assessed at Thames Hospital and was discharged soon afterwards.
Mr McDonald said any traumatised students will be offered counselling tomorrow.
A 14-year-old will appear in the Hamilton Youth Court on a number of charges including firearms offences.