A schoolboy with a dairy allergy was left gasping for breath and shouted “I’m going to die” minutes after a fellow pupil flicked cheese at him, an inquest has heard.

Karanbir Singh Cheema, 13, known as Karan, went from “absolutely fine” to unconscious in under 10 minutes after the incident at William Perkin Church of England high school in Greenford, west London, on 28 June 2017.

Karan, described as “so bright he could have been anything he wanted”, died 10 days later at Great Ormond Street hospital.

Lucjan Santos, who was a teacher at the school, told the inquest at St Pancras coroner’s court in north London that Karan approached him as break was ending to ask him if he knew of a particular pupil, whose identity cannot be reported for legal reasons.

“He [Karan] said: ‘He put some cheese down my collar for no reason,’ and then he pointed towards the back of his neck,” Santos told the court. The teacher said he told Karan that that was not a nice thing to do, at which point the teenager said: “And I’ve got a cheese allergy.”

Santos said he sent Karan to see the school’s welfare officer and to fill in an incident report. Describing the boy’s demeanour at that point, the science teacher said he “appeared fine and spoke calmly”.

Karan had multiple food allergies, including to wheat, gluten, dairy products and nuts. He also had asthma and atopic eczema.

The boy who flicked the cheese and another pupil who passed it to him, both aged 13 at the time, now aged 15, told the inquest on Wednesday they were unaware Karan had an allergy to the food.

When asked whether pupils at the school were aware of the potentially fatal consequences of allergies, Santos answered: “Hopefully.”

Bonny Campbell, a school administrator with first aid training, said that when Karan first arrived in the welfare office he was scratching his neck but sat down calmly, but he became increasingly panicked, “leaning over the sink, gasping for air, still scratching his neck”.

Campbell said blood was seeping through Karan’s shirt from the scratching and he was lurching around the room. “He pulled his shirt off, he was very itchy and couldn’t breathe,” she said. “He had scratch marks on his neck and his stomach, he was screaming.” She acknowledged that at one point he was shouting: “I’m going to die.”

Staff gave Karan Piriton, an over-the-counter allergy relief medicine, and his inhaler. His condition deteriorated and they then administered his EpiPen. Campbell said it appeared to have no impact as he went “quite grey” and limp and his eyes glazed over. The EpiPen used was a year out of date, the inquest heard.

Asked by the senior coroner for inner north London, Mary Hassell, as to why a second EpiPen was not administered, Campbell told the court that in first aid training she was told to wait 10 minutes before another dose, that an ambulance was on the way and that the emergency operator had not advised staff to do so. She also said the first EpiPen did not have the desired effect and staff would not use another student’s device, which would have been the only option for administering a second dose.

Dr Vivien Chan, a consultant paediatrician who counted Karan among her patients, said she believed the usual advice was to administer a second EpiPen five minutes after the first.

She also suggested the second dose could be administered as soon as possible if deemed necessary but said she could not tell whether, in Karan’s case, a second EpiPen would have made a significant difference.

Chan said it would be wise for schools to always hold copies of pupils’ allergy action plans; at present, it is usually left to parents to pass them on.

Proceedings are expected to conclude on Friday.