"It's like an epidemic because the children have been dropping like flies. We don't know what is causing it."
She said other children had suffered from swollen hands and feet.
Dr Rachel Heathcock, from the health protection unit, said: "We can confirm that the unit is aware of cases of probable viral illness at a school in South-east London.
"Cases of viral illness are to be expected during the winter months. The unit is investigating the cause of this illness, and results of samples from a number of children will be available in around a week."
She said the unit had provided the school and parents with advice on reducing the spread of infection, including the importance of good hand hygiene and keeping ill children at home until 48 hours after symptoms have passed.
She added: "Those cases who are feeling particularly unwell, or whose symptoms have lasted more than 48 hours, have been advised to contact their GP."
Headteacher Val Hughes said: "Here at Sandhurst we take the health of our children very seriously and acted as soon as we realised there was a sickness bug at the school.
"Fortunately the bug seems to have passed and school is returning to normal."
The school is not alone in suffering from the so-called winter vomiting virus.
Lewisham Hospital has seen an increase in admissions of people suffering from that kind of complaint in recent weeks.