Thirteen school pupils were taken to hospital after eating 'sweets' containing the chemical THC found in cannabis.
Five ambulance crews were sent to La Sainte Union Catholic School in Highgate, north London, after the teenagers fell ill on Monday morning.
The youths were treated at the scene before being taken to hospital but none of the youths were "seriously unwell", according to police..
Their parents have been informed of the incident.
The secondary school, which is primarily teaches girls aged 11 to 18 but has a mixed sixth form, said in a statement: "The students became ill after eating what they believed were sweets.
"The contents of what the students ate and how they came into possession of them is being investigated by the police."
Local officers in the borough of Camden said a few hours later that they understood the sweets contained THC, the main psychoactive component of marijuana.
"We await tests to establish the quantity of THC in each sweet," Camden Police tweeted.
"While we await a full update from hospital, nobody is seriously unwell. There has been no arrest. Enquiries continue."
La Sainte Union received a “good" rating in its most recent inspection last year and its former pupils include actress Imelda Staunton and singer Tulisa Contostavlos.
The London Ambulance Service said they received the call to the school at 11.44am.
"We dispatched a number of resources including a clinical team leader, five ambulance crews, medics in cars and incident response officers," a spokeswoman said.
"We treated thirteen teenagers at the scene and took them to hospital."
Police also confirmed the school had not been evacuated as a result of the incident.
The Metropolitan Police said in a statement: "Police were called by London Ambulance Service at around 11.45am on Monday, 5 October, to reports of children feeling unwell having eaten what they believed to be sweets at a school in Highgate Road, NW5.
"As a precaution, 13 children have been taken to hospital. Their parents have been informed."
