One of the children suffering from the potentially fatal infection was described as serious but stable in hospital last night. The eight-year- old girl is a pupil at the school in Macduff, north-east Scotland, where the outbreak started last week.
Dr John Curnow, consultant in public health medicine for Grampian Health Board, said more than 100 people have been tested for E. coli O157 in the area. He added: "There are now 30 positive individuals - which include 22 from the class and three adults directly or indirectly involved with teaching the class.
"The remaining five positives are siblings and parents of pupils in the class." A 10-year-old girl, who is said to be a secondary case, is in a stable condition in hospital in Aberdeen.A seven-year-old boy was discharged from hospital yesterday.
Grampian Health Board officials say the outbreak almost certainly started with a batch of home-made goats' cheese taken into the school as part of a class project.
Dr Curnow said: "We are 99 per cent certain that it was the cause but the cheese has all gone. But we do have the goat and it has tested positive.
"Our main concern now is to ensure that secondary spread is kept to a minimum."
In a separate case, two sisters aged two and three, were under observation in hospital in Kirkcaldy, West Fife, after being admitted on Sunday showing symptoms of E. coli O157.