A 16-year-old girl reportedly baked the brownies at home before taking them to Shirley High School in Croydon, south London, according to the Sun newspaper.
Emergency services said they were called at around 3pm on Friday March 29 after "a number of students reported feeling unwell".
The Sun reported that police officers had spoken to the girl, who is unlikely to be expelled.
A Metropolitan Police spokeswoman said: "Police were called at 14.56hrs on 29 March to a school in Shirley Church Road, Croydon, after a number of students reported feeling unwell.
"Officers attended with LAS (London Ambulance Service).
"Three pupils have been taken to hospital. None are in life-threatening condition."
The force has made no arrests and is "working to trace a number of other pupils", it added.
Shirley High School Performing Arts College was not immediately available for comment.
The co-educational comprehensive school has just over 1,000 students and first opened in 1954, according to its website.
It was awarded specialist status as a performing arts college, it added.