Details of the nature of the incident have not yet been revealed, but parent Mark Wale tweeted that the school had notified families of the evacuation and that children were being sent home.
He said police officers had escorted children from the school in small groups.
A message he received from the school 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. Ppease collect them from Birley Lane main entrance if you are able to. Please do not call school."