Six boys and one girl shared the brownie at lunchtime, according to authorities. The 6th-grade students are 11 and 12 years old.
Paramedics were called when the first child started feeling dizzy, nauseous and had difficulty breathing. The others suffered the same symptoms. They were taken to Hoag Hospital and were expected to be OK.
One of the kids told authorities that he shared the brownie with the others, telling them it was laced with pot.
"When one kid took a strange bite and said 'You know, this doesn't quite taste like a regular brownie,' and he said 'Oh by the way, this is laced with marijuana.' So he was pretty forthcoming and told them what it was," said Fred Seguin, acting chief of the Costa Mesa Fire Department.
It was not immediately clear where the student got the brownie.
Police did take a backpack and the remainder of the brownie as part of the investigation.
School district officials said they will decide if any students should face disciplinary action. Officials say that will depend on whether the children knew what they were eating.