The principal of a Hamilton school where a teenager was stabbed is asking people to delete video footage of the incident.

The 16-year-old was left with his thumb hanging by a thin thread of skin after an incident outside the school's Waimarie St entrance on Friday afternoon.

The boy's mother, Pauline Hania, was shocked by the attack, in which she claims a student tried to kill her son.

"There was an altercation and this kid actually tried to kill him," she said. "He tried to get him in the stomach, my son has cuts on his shorts from the knife," she said.

Her son needed four hours of reconstructive surgery.

While principal Virginia Crawford hasn't responded to media requests for comment, she did this afternoon issue a school newsletter addressing concerns from many parents.

"I know that our school community will be concerned for the welfare of the young people directly involved and their families," she wrote.

"The parents of the injured young person informed the school that their child is recovering as best as they're able to under the traumatic circumstances they've experienced."

She then asked people with video footage of the assault to delete it.

"If you know that your child has a recording of the incident, please delete the recording and do not share it on social media, as that action will retraumatise the young people and their families, and I'm sure you will understand that right now both families are focusing on looking after their child."

She also addressed why a student was able to bring a weapon to school and said some situations were "escalated as a result of students engaging in social media ... or miscommunication of tone and intent, and sometimes culminating in an arrangement to meet, leading to a serious incident, face to face".

When asked about why she alerted parents so late on a Friday night - 9.30pm - she said prior to that time, school management was busy with police and "accurately assessing the situation and why and how it happened".

The school wasn't put into lockdown as the alleged attacker accompanied school staff and was "contained immediately" after the incident.

"The police arrived at the school soon after and managed the perpetrator. The school nurse administered appropriate medical attention whilst waiting for the ambulance to arrive," she wrote.

She added that updates in regards to the two teens would only involve their parents.

The school was following its "disciplinary procedures" in regards to the student, she said.