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.