Mr Oppatt added: “When we arrived we saw Karan lying on his back on the floor with teachers around him.
“He appeared to be in a state of pre-arrest. He had very slow respiration – he was gasping for air. His skin was red and there appeared to be hives.”
Realising the severity of the situation, Mr Oppatt knew he had to call for help but had to go outside because his radio could not get reception.
When he returned, his colleague told him their patient had stopped breathing, so they started CPR, gave him adrenaline and used a defibrillator while they waited for backup.
When additional crews arrived, Karan was taken on a stretcher to the ambulance, but he never regained consciousness and died with his parents at his hospital bed in Great Ormond Street Hospital ten days later on July 9.