Emergency services have locked down South End School in Carterton, Wairarapa, after a mystery chemical fell from an aircraft.
Fire, ambulance and police responded to incident at 2.30pm and a rescue helicopter was dispatched from Wellington just after 3pm.
One student reportedly told school staff they had seen a plane dropping a substance near the school, and a strong smell of rotten eggs was reported in the playground.
At least five ambulances and eight fire appliances are at the school.
Carterton senior fire officer Wayne Robinson said 33 children had shown signs of illness, including vomiting, and four would be taken to hospital.
It is now planned to shower all staff and students. Parents have been asked to bring a change of clothes for their children.
No students are currently allowed to leave the school grounds, and no one is permitted on to the grounds.
South End School is a primary school with 140 students from year 1 to year 8.