Kavanagh College 1st XV coach Ant Harris said Matt was halfback for the side.
On Saturday, his team were playing against King's High School 2nd XV in the under-18 grade at Dunedin Rugby Football Club.
With about three minutes of the match remaining, Matt was tackled across the chest in a "hard, legitimate" tackle that caused a sudden whipping to his head.
"It was his head going back and hitting the ground that was the impact that caused some bleeding to the brain."
The referee saw the impact and stopped the game, Mr Harris said.
"He [Matt] was okay immediately after. He was on the ground but not knocked out. The physio came out and asked him a few questions - what the date was and what the score was - and he answered those fine. He knew where he was and he knew what the score was.
Matt was wearing headgear.
After the tackle, Mr Harris went on the field to check on Matt, who was joking with the King's players.
But minutes after passing the concussion test, he collapsed.
"He was about to put the ball into a scrum. As he went to bend over, he fell to his knees and there he stayed. He was concussed from that moment on."
The match was called off.
The incident had been difficult to watch, Mr Harris said.
"Not just me, for all our boys, it's pretty hard."
The school had provided a support group for pupils on Sunday.
"We got everyone together, we just need to be around each other at the moment. It's bloody tough."
The talented halfback had a leadership role in the team.
"The boys look up to him and he takes it on himself."
He was hopeful Matt would make a full recovery with minimal long- term impacts from the injury.
