A parent said a Tangaroa supporter had run onto the field and punched an Otahuhu player from behind.
The 15-year-old boy's condition was originally described as serious, but police said he had since improved and was talking with them at hospital.
He is now described as having moderate to minor injuries.
A spectator at the game called in to Veitchy on Sport this afternoon said it was "complete chaos".
He alleged the fight started after the Tangaroa player charged the Otahuhu player.
"They threw a couple of punches and pushing each other and then from how I understand it a relative from the Tangaroa student, 'cause the fight took place where the Tangaroa supporters were, and he just threw a punch into the back of the Otahuhu student's head and knocked him completely out cold," the man said.
The man then told Veitch that the Otahuhu students all ran in and started fighting.
"It's just shocking, watching everything unfold. A referee got between everyone and stopped it, and sent everyone off to their sides. Otahuhu went to their side, Tangaroa went to their side," the man said.
"And from what I understand somebody from the Otahuhu side went over to Tangaroa and they all started fighting, and then all these students where fighting and parents also getting involved as well, hitting students with umbrellas. It was just really bizarre to be honest."
Police said they had spoken with a number of witnesses and were investigating who was responsible for the boy's injuries.
Counties Manukau senior sergeant Jeremy Field said the behaviour was unacceptable.
"This sort of behaviour is not tolerated in the community," he said.
"Everyone who attends sporting events like this needs to be safe and feel safe while attending."
Police had spoken with several people in relation to the incident but weren't yet sure of the cause of the fight, he said.
He said it was unclear whether any charges will be laid and that the investigation was ongoing.
Field was unaware of any previous tension between the teams.
Both schools have been co-operating with police enquiries so far, he said.
He added that it was not clear whether school rugby brawls were becoming a trend.
Two weeks ago, and 11-year-old boy allegedly punched a referee after tackling him to the ground during an under-12 rugby game in Papatoetoe.
The incident happened about 10 minutes into the second half of a rugby game at the Papatoetoe Sports Grounds.
The child could face a lifetime ban from the sport.