A pupil at Alderley Edge School for Girls was left feeling unwell after eating a brownie which is believed to have contained cannabis.
Cheshire Police are investigating the incident which occurred on the school site on Wednesday, 24th February.
It is understood that four students were given chocolate brownies containing the drug by two other pupils. As a result one girl had to be collected by her mum and taken home after becoming ill.
The mother, who is a frontline worker and wishes to remain anonymous, said "When I got there she wasn't well at all. I took her home. She looked dreadful. At 6.30pm a teacher called saying there had been cannabis found in the brownies."
Headmistress Nicola Smillie said "Alderley Edge School for Girls has a zero tolerance policy to drugs and alcohol and we contacted the Police regarding an alleged incident involving drugs on the school site yesterday.
"We are unable to comment further. Schools across the country have thorough policies in place for such situations and the school will be working closely with Police and parents on the alleged isolated incident."
A spokesperson for Cheshire Police said "We are aware and are working with the school to establish the circumstances."
At the time of publication we are awaiting an update from Cheshire Police who we understand were visiting the girl today for an "informal discussion".