Two teenagers were taken to hospital after taking drugs a student found in a park.
The drugs were suspected to be synthetic cannabis or hashish.
Police told Fairfax a 14-year-old student discovered a packet containing six pills in Thorp Bush reserve near Motueka High School, took them to school and handed some out.
Senior Sergeant Scott Richardson of Nelson Police said two of the pills were ingested, "giving one student a severe reaction and one a moderate reaction".
"Any time young people get hold of pills ... and ingest them without knowing what they are, that's a real concern," he said.
Sergeant Rob Crawford of Motueka told Fairfax the students had described the drugs as being in capsule form, and police suspected them to be a Class C drug. Based on the description it was possible they were synthetic cannabis or hashish, he said.
Toxicologists were working to establish what the drugs were.
Motueka High School Principal John Prestidge said the incident took place near the end of the school day almost two weeks ago.
The students became ill on their way home and he understood a GP later called an ambulance, according to Fairfax.
Two of the three students involved were back at school and the Board of Trustees was considering the best course of action for the remaining student, Prestidge said.