He said there were just 200 pupils at the secondary school and everyone had endured more than their fair share of trauma in recent times.
"They've had the earthquakes and Covid and now [lost] one of their peers," he said.
Kaikoura High School principal John Tait said Sebastian's death was incredibly sad.
"This has been a big shock and loss to both Kaikoura High School and Kaikoura Suburban School, as well as our community.
"Our heartfelt condolences go out to the family," he said.
Police said they attended an incident at a Factory Rd, Hapuku property just before 2.30pm after a person had been injured.
They were taken to Kaikoura for medical treatment however sadly died.
The death will be referred to the Coroner.