A marauding group of youths have descended on a rival Otago school two nights in a row, leading to a brawl which put one young man in hospital with serious injuries.
The "deplorable actions" have cast doubt on the future of one of the longest-running First XV rugby matches in the South, St Kevin's College principal Paul Olsen says.
St Kevin's is due to play Waitaki Boys' High School for the 86th time next Friday.
On Wednesday night, a group believed to be made up of about 30 former Waitaki Boys' students entered St Kevin's grounds.
An altercation ensued between St Kevin's students and the group, resulting in one person aged 18 or 19 being admitted to hospital with "relatively serious injuries", Sergeant Tony Woodbridge, of Oamaru, confirmed.
The injured male was neither a school student nor from Oamaru, Woodbridge said.
It was the second successive night a group had trespassed on to the St Kevin's precinct, vandalising property and "traumatising" staff families, Olsen said.
"We are saddened by these events and, in response, have locked the gates and are going through our video footage for identification of faces and number plates.
"Police and high schools have been in communication over the past week to try to mitigate issues concerning the behaviour of ex-pupils both at this traditional interschool game and before the event.
"Regrettably, the deplorable actions of these individuals seriously call into question the future of next Friday's match."
While there were no reports of current Waitaki Boys' pupils involved, school rector Darryl Paterson said he "fully supported" Olsen's stance.
Woodbridge said police were "upset and angered at those actions from both sides".
"Police will take positive action and arrest and charge people where appropriate."
The game, known as the Blood Match, is usually a highlight of the North Otago sporting calendar, but has attracted controversy in the past, generally from incidents off the field.
In 2018, police investigated after a Waitaki pupil was hit by a car on St Kevin's grounds two days before the game, and a sideline scuffle led to an arrest during the 2015 clash.