Downsview Secondary School near Keele and Wilson was evacuated Friday morning after water from its swimming pool flooded the mechanical room.
Just before 10 a.m., a power outage caused a pool pressure valve to fail and flood the mechanical room with water and chemicals.
A hazmat team was called to the school over concerns chlorine from the pool could mix with a tank containing acid.
Toronto Fire sent 13 trucks to the scene.
No injuries, or any fumes have been reported.
The students were allowed back in around 11:30 a.m.
The pool will be closed for several days.