Students were rushed to hospital after smoking a "dangerous substance" thought to have been added to an e-cigarette.
Paramedics were called to Saddleworth School in Oldham, Greater Manchester at around 8.25am on Monday morning after a pupil became unwell.
Less than three hours later the emergency services were called back to the school after more children reported feeling sick.
Although it is not clear what the substance was, a statement from the school warned how "dangerous, unacceptable and potentially life changing" the incident could have been.
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It read: "A number of pupils have been taken ill due to the inhaling of a dangerous substance that appears to be in circulation in our community.
"The substance in question has been added in an e-cigarette. The risks of taking such substances are obvious and potentially very serious.
"We have today met with the entire cohort concerned to re-iterate how dangerous, unacceptable and potentially life changing such behaviours are."
The school said a large emergency service response was needed to care for the sick kids.
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"By way of context, we have had to call three ambulances that could have otherwise been responding to genuine medical emergencies and accidents," they added.
"We would call upon everyone in our community to be vigilant and work together to help us track down how this dangerous substance is being distributed to children.
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"Parents and carers are asked to make it clear to their children that the consumption of such substances is harmful.
"We are working with the police to track down those responsible."
They urged anyone with information leading to the supplier to contact the school.
