A 16-year-old needed reconstructive surgery after his thumb was left hanging by a tiny piece of skin.
The boy's mum claims her son's attacker tried to kill him during a fight outside Hamilton's Fraser High School on Friday afternoon.
"There was an altercation and this kid actually tried to kill him," Pauline Hania 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.
She had initially been displeased with the response from the school, but today told the Herald they had been "more helpful" yesterday.
Police had attended the incident.
The alleged attacker had been referred to Youth Aid.
There were bloodied photos of the boy's injury as well as videos of the fight itself.
A photo of the thumb shows it displaced and hanging on by a piece of skin.
A letter to parents on Friday night said a "serious incident" occurred outside the school gate on Waimarie St.
"The police and ambulance were called immediately. The school and the police are investigating the incident.
"A senior male student and another male youth were involved in a serious assault. The youth was injured and given medical attention," principal Virginia Crawford wrote.
"I'd like to thank Mr Ross Hope and Mr Simon O'Connor for being present at the time, and acting quickly, calmly and safely."