Police would not comment on the nature of the alleged assault.
Kaikoura High School principal Werner van Aswegen said he had no idea what led to the incident.
"The injuries sustained were not serious, but they are still being taken very seriously by the school as it is totally out of the ordinary for us," he said.
"We actually have to deal with something which we normally don't deal with. It is disappointing, to say the least."
Mr van Aswegen said the school had systems and procedures in place to provide for the safety of staff and students in such situations.
Those systems were put in place and students were offered counselling.
The two teachers returned to school on Friday but the student will remain suspended until the board of trustees can convene a disciplinary committee to decide his future.