At least 35 children were examined by medical teams after police evacuated more than 500 students and staff from the school a few kilometres east of the city of Zurich.
Police received a report at 10.30am from the school that an unknown substance had caused respiratory problems in several children.
After the teaching staff and pupils had been safely evacuated to the neighbouring gym, chemical specialists from the army searched the building and examined the air for pollutants. "Up to this point, no harmful substance could be detected," communicated Zurich police.
The children who had been physically examined were released soon after and the school reopened three hours after it was closed. The cause of the respiratory problems remains unknown, said police.
In February 2016 a school near Geneva's diplomatic district was also evacuated after police received a bomb threat.