Pukekohe North principal Robyn Withers-Lauer said the scheme found 41 of the school's 190 students had untreated skin infections in its first week.
"At this time of year when it's getting near the end of term in winter, skin infections do tend to be higher," she said.
Dr O'Sullivan's sister Nikki De La Rosa, who lives locally and coordinates the scheme at the school, said a third of the children who joined it were not registered with any family doctor, partly because they moved often.
"Transience is massive," she said.
Volunteers school receptionist Sarah Cole and school trustee Denise Proctor said the high infection rate also reflected overcrowding in many homes, with many parents on benefits and in cold houses.
"They are not insulated, they are not healthy homes," Mrs Cole said.
She said families often could not get appointments with family doctors for two days.
Some did not have cars, or had only one car that was used by the father to go to work.
David Tautari was in bare feet when he came to have his sores checked, and Mrs Proctor said most of the children went barefooted even in mid-winter. Mrs Withers-Lauer said that was "the child's choice".
"We give them shoes. We have KidsCan shoes and socks and jackets and pants, but they tend to like to run around in bare feet," she said.
Dr O'Sullivan said he contacted the school when he was in Pukekohe to visit his dying father late last year and saw a child in a park with untreated sores on his stomach.
He explained his scheme at a public meeting three weeks ago and 160 parents turned up. More than 80 per cent of pupils have joined the scheme.
Tauhoa School, a small 40-pupil decile 6 rural school near Wellsford, also joined the scheme three weeks ago after its principal Vivienne Goldsmith approached Dr O'Sullivan at a Northland principals' conference. It has found 10 children with untreated skin infections so far.
A Ministry of Health spokeswoman said the scheme was funded for four years until June 2017 as a pilot under the Maori Health Innovation Fund.
"Any considerations for future funding to extend (or spread) the programme will be dependent on the evaluation results due after completion of the contract."