Students were sent home from Birley Academy, Birley, after a 'suspicious incident' was reported to South Yorkshire Police at 9.40am today.
No other details have yet been revealed but police officers remain in the school and its grounds.
A police spokesman said: "Officers were called to Birley Academy in Sheffield after police were alerted to a suspicious incident at 9.40am this morning.
"The college chose to immediately evacuate students as a precautionary measure and officers remain at the scene."
A message sent out by the school to parents today said: "Unfortunately there has been an incident at school today and the building has been evacuated.
"It may be some time until we can return, students are being sent home. Please collect them from Birley Lane main entrance if you are able to. Please do not call school."
A Year 9 student said there is concern at the lack of information released.
He said: "We were evacuated in the first lesson and were standing outside for one hour straight. Some of the other kids were saying police were outside the school, I went to check outside and had seen they’d blocked all the entry points to the school off.
"Police were not speaking on the matter to parents and saying that they couldn’t release information. People should have the rights to know why the school has been shut down."