A teenager was rushed to hospital and several others taken ill after taking a Class B drug at a school in Harlow on Thursday.
A 13-year-old girl was arrested on suspicion of drug dealing and has since been released under investigation.
Essex Police said they were called to St Mark’s West Essex Catholic School at 12:30pm after reports of eight teenage girls had taken a Class B drug.
Two girls were treated by paramedics at the scene while the one who was taken to hospital. She has since been discharged.
A statement made on behalf of headteacher Elaine Heaphy, said: “A pupil brought a substance into school. Subsequently, a small number of pupils felt unwell and notified school staff. They were taken to hospital as a precaution.
“This unprecedented incident was dealt with quickly and efficiently, in accordance with health and safety procedures; the safety of pupils was paramount at all times.”
A full investigation by the school is now taking place.