When he attempts to lift himself up on his feet, two boys pull the wheelchair out from underneath his legs, and he falls hard to the ground.
A girl behind the camera takes a shocked gasp, before moaning: "Oh my god."
But Mr Couillault said the video was cut before it could show the end of the incident -- when the teens realise what's happened and pick the boy up and take him to the school nurse, he said.
The confrontation between the boys -- all in Years 9 and 10 -- was simply a school-yard fight over a girl, Mr Couillault said.
"The boy in the wheelchair actually wheeled up to the other guys and sort of got into their personal space, they were making an effort to create some personal space, and it went beyond that to another level," he explained.
"It's schoolboys trying to resolve a matter and it doesn't go well, which happens all over the country."
But "the minute he had the fall, they say, 'oh sh**, sorry bro', and pick him up and take him off to the nurse", Mr Couillault said.
"It wasn't, 'ha ha' and walk away. It's, 'sh**, we've got this wrong, and we're aware of it'."
The boy in the wheelchair -- who suffers a musculoskeletal degenerative condition, and wears leg braces for support -- was "absolutely fine" and had suffered "zero physical damage" in the incident, Mr Couillault said.
The three boys had made amends.
However, since the video hit the internet yesterday, the two boys and the girl who filmed it had suffered a torrent of abuse on social media.
The two boys had left Auckland due to their safety fears, and the girl had been hospitalised.
Mr Couillault said the incident showed the dangers of people on social media making "half-arsed value judgements about a situation they don't have all the facts about".
The family of the boy in the wheelchair was "totally happy with the way we dealt with it", Mr Couillault said.
"They didn't want the kids kicked out. They're happy with how we dealt with it, and understand their son can get into arguments with people sometimes. They just want him treated like everybody else."
He was in more danger of suffering emotional harm from the media scrutiny of the incident, Mr Couillault said.
"The social media scrutiny and people passing judgement and making threats without having the whole story is just galling.
"It's so dangerous when you don't have the full picture and accuse people of things and pass judgement and make threats ... you're putting people's lives at risk."
The social media storm had "created so much anguish", he said.
He was "really concerned" about the two boys and girl from the video, he said, "for their well-being, for their families' well-being".
In a post on the school's Facebook page, Mr Couillault said management had dealt with the incident "swiftly and seriously".
"It happened on Tuesday, and the victim is at school and happily engaged with his friends," he said.
"The leadership and staff at the school agree that this behaviour is totally unacceptable on every level, but humbly request that you trust us to work with both families to create solutions that work ... as we have begun to do."
Where to get help
• Lifeline: 0800 543 354 (available 24/7), ww.lifeline.co.nz
• Suicide Crisis Helpline: 0508 828 865 (available 24/7)
• Youthline: 0800 376 633, www.youthline.co.nz
• Kidsline: 0800 543 754 (available 24/7), www.kidsline.org.nz
• Whatsup: 0800 942 8787 (1pm to 11pm), www.whatsup.co.nz
• Depression helpline: 0800 111 757 (available 24/7), www.depression.org.nz
• Samaritans: 0800 726 666 (available 24/7), www.samaritans.org.nz
• If it is an emergency and you feel like you or someone else is at risk, call 111.
