Emergency services are heading to South End School in Carterton after a number of students have fallen sick.
The school said a strong smell was discovered in the playground at about 1pm and several students have reported feeling dizzy and unwell.
One student told staff they had seen a plane dropping a substance near the school, the school said in a Facebook post.
Staff are monitoring the students closely.
No students are allowed to leave the school grounds and nobody is allowed to enter the school until emergency services have investigated the substance.
Wellington Free Ambulance Service has an area manager at South End school assessing the situation, plus four ambulances attending to the students
Fire services said they sent four crews from Wairarapa to an incident in Carterton, with support vehicles from Wellington and Hutt Valley.
South End School is a primary school with 140 students from year 1 to year 8.
More to come.