Emergency services – including two air ambulance helicopters – attended the school shortly after 11:20.
Lisa Barrett, whose daughter attends the school, told the her daughter texted her to say the school was in lockdown.
“It is really worrying,” Ms Barrett had said.
“You hear from your child saying someone has been stabbed you don’t know what to do.”
Hundreds of parents then lined the school gates waiting to hear news of the safety of their children locked inside.
It was not until about 15:20 that the secondary school announced the situation had been “contained” and police began letting children leave.
