Carrickfergus Grammar School pupil Ben Robinson was not taken off and continued to play during the Medallion Shield contest against Dalriada High School in January.
He collapsed after a tackle and never regained consciousness.
One fellow player said: "He is clean out there."
The schoolboy was treated at the scene by Dr Paul Loan and taken to the Royal Victoria Hospital in Belfast, where he was pronounced dead.
The inquest heard on Wednesday from his father Peter, who spoke of his concerns as footage showed Ben getting up after a series of bruising tackles holding his head.
"He was hunched, he was not his normal body position, you could see that," he said.
The inquest saw him talking to a team-mate on the pitch - family lawyer Gabriel Ingram said the other boy was looking "quizzically" at the victim.
He added: "He appears to have his head off to the right and forward slightly."
Mr Ingram said police investigating the incident drew a blank. An officer said it was only when lawyers for Ben's family intervened that important evidence came to light.
Constable David Mannis took a number of witness statements but his initial appeal to the principals of both schools for pupils at the match to come forward had been unsuccessful.
Mr Ingram said: "The family's dissatisfaction with the investigation by Constable Mannis - that effectively it was in bits and pieces throughout and that he took a number of statements and considered things to be closed but then there were further material witnesses put forward by the family and Constable Mannis thereafter took their statements and they have added in their opinion significantly to the truth of what happened on that football pitch."
The officer was told by school principal Tom Skelton that pupils at Dalriada were too traumatised to give evidence.
Mr Ingram told the policeman: "You have lost a huge potential chunk of witness evidence by not pursuing it."
Mr Ingram asked if the officer thought that was enough evidence to verify what he thought had happened without further pursuing it.
He said witnesses gave important evidence about Ben's injuries and demeanour as well as suggesting that he should be taken off the pitch - but this wasn't followed up by police.
Following Mr Ingram's intercession, 300 letters were sent to parents at Ballymena Grammar, and they elicited five or six new witness statements.
Mr Ingram said: "Before my input the net effect of your investigations into this case in terms of trying to get witness statements from the pupils is that you had drawn an absolute blank as regards Dalriada and Carrickfergus Grammar School. You had drawn a blank in terms of leaving it in the hands of the headmasters."
The policeman was also forced to liaise with the family. He admitted there were "difficulties" in his relationship with Ben's police officer mother.
He has known her for some time and drove her to work while they were in training, but had never alerted his superiors to the issues.
Mr Ingram asked if he was out of his depth, which he denied.
"I was not going to walk away and throw the towel in," he said.
Mr Ingram said he should have explored inconsistencies between a witness statement from Dr Loan and his apparent appearance on the pitch in a video recording of the match.
Mr Skelton said he had followed the officer's instructions.
"I did what I thought was right, subject to school policy," he said.
"I was asked to ascertain if any of the pupils were prepared to give evidence and I carried out that request."
He added: "They felt they had nothing that would be of any benefit regarding the inquiry - their own focus was on the game of rugby."
The inquest continues on Thursday.