One teacher and 24 students were sent to receive medical care after someone noticed gas was spreading through the Rudskolan school in Karlstad, southern Sweden.
"We don't know yet what this is and we're working on finding the source," Tommy Sjöberg of the Karlstad police told the TT news agency.
He added that pupils were complaining of headaches and feeling nauseous. Some of the students were also coughing and some suffered chest pains.
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Fire fighters and police were sent to the scene to comb every room in the hope of finding an answer.