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."