A 13-year-old girl has been arrested at a school on suspicion of having drugs with intent to supply after a number of pupils fell ill.
Police were called to St Mark's West Essex Catholic School in Harlow on Thursday to reports that eight girls had taken a class B drug.
The force said two of the girls were given medical treatment by paramedics, and a third was treated in hospital and has since been discharged.
The school said all of the pupils affected by the "unprecedented incident" were discharged from hospital on the same day without the need for further medical attention.
A teenager was arrested on Thursday on suspicion of possession with intent to supply class B drugs, and has since been released under investigation.
A statement from the school said: "On Thursday 28 February, 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. All were discharged the same day without need for further medical attention.
"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 into the incident will now take place."
A spokeswoman for Essex Police said: "A 13-year-old girl from Harlow arrested yesterday, February 28, on suspicion of possession with intent to supply class B drugs has been released under investigation.
"We were called around 12.30pm with reports that eight teenage girls had taken a class B drug.
"Two girls were given medical treatment by paramedics, whilst a third was treated at hospital and has since been discharged."