Students have been kept in isolation at Wanganui Collegiate School after a nasty gastro bug hit the premises.
Whanganui Public Health medical officer Patrick O'Connor said about 100 students had been unwell since Sunday, mainly with vomiting and some diarrhoea.
"We do not know the exact cause, but a virus is likely. Lab tests may give us answers by early next week," Mr O'Connor said.
He said viral gastroenteritis circulates in the community, especially in the winter months.
"It spreads most rapidly in settings such as rest homes and boarding schools where people live in close proximity," Mr O'Connor said.