Thirteen teenagers were treated by paramedics at La Sainte Union Catholic School in Highgate, north London, on Monday morning before being taken to hospital.
Police said the sweets are understood to have contained tetrahydrocannabinol, known as THC, which is an active ingredient of cannabis.
Investigations are under way to establish the quantity of THC in each sweet, the Metropolitan Police’s Camden branch said.
None of the teenagers are "seriously unwell", the force added.
Police and London Ambulance Service attended the school shortly before midday following reports children had become unwell having eaten what they believed to be sweets.
The Met said they were taken to hospital "as a precaution" and no arrests had been made.
A spokesman for the school said: "A small number of our students have been taken to hospital today after becoming ill.
"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.
"We have made parents aware of this incident."