The comments were made after Dunedin pupil Enere McLaren-Taana, 16, died after an attack at the city's bus hub on Thursday afternoon.
A 13-year-old boy who is accused of his murder appeared at the Youth Court on Friday.
Police found a knife at the scene and are investigating the circumstances leading up to the stabbing.
In Auckland, there have been a spate of incidents involving knives, and there was one fight where a machete was used.
Secondary Principals Association president Vaughan Couillault said violence between pupils was becoming more acute, leading to serious injuries or worse.
Fights were often among groups and they were "not stopping when the person goes down", he said.
Anecdotally, those engaging in fights were "harder to bring back from the edge - it's harder to de-escalate".
Couillault said there was also an "increasing likelihood that some sort of weaponry is involved".
Family members and friends of students were also getting involved in fights. In the machete incident, the weapon was brandished by a member of the community, not a pupil, he said.
"What we are seeing is the willingness of people who aren't students at school to get involved in what were, 30 years ago, schoolyard conflicts that resolved relatively swiftly and easily. [They are] becoming far more complicated because of the involvement of the wider community."
Umbrella Wellbeing chief executive Dougal Sutherland said the apparent involvement of a weapon was shocking and concerning.
Historically, teenagers had fought with their fists and while this was not to be condoned, the use of a weapon was "really, really troubling", he said.
Violent incidents at bus and train stations involving young people did seem to be on the increase, he said, but there was no obvious reason for this.
However, those types of incidents rarely happened in isolation, he said.
"There's a lot of contextual implications that need to be taken into account, as well as the developmental level of a [young person].
"A lot of factors will have combined to produce this really terrible and tragic outcome."
Overall, offending by adolescents had decreased in the past decade - but a certain group of teenagers were offending more, Dr Sutherland said.
"For a specific subset of adolescents - those with more pervasive antisocial behaviour present since childhood - rates have increased as they are engaging in more offending behaviour.
"These are likely to be the group who show up in the spikes of offending [such as] ram raids, violent behaviour at bus stops and so on."
