Three 15-year-olds are under investigation for allegedly attacking a fellow student at James Cook High School in south Auckland.
The student, a 16-year-old boy, was taken to Middlemore Hospital with serious head and facial injuries, and is now in a stable condition.
Suspension is likely as the school's Board of Trustees decide on the future of the bullies.
- Auckland teen seriously injured after bullying incident
"The student who was bullied was foot-tripped and then punched once and fell to the ground," explained principal Grant McMillan.
"It was a sealed area and he hit his head - our duty staff were there moments later, and student prefects were there as well."
He said the students "will be held responsible within our school's disciplinary system".
"The students who have broken our expectations... we have a very simple response. Serious bullying involves police because we are trying to grow fine young citizens here," he said.
The incident comes less than a fortnight after national anti-bullying day Pink Shirt Day.
The school doesn't believe Monday's incident is an on-going feud.
"What I think, is this is probably some falling out stuff over the weekend - we still have to do our work on that, but there doesn't appear to be a pattern in this," Mr McMillan said.
While the victim has been discharged from hospital, police are still involved in dealing with the bullies.