Guildford Grammar School was locked down about midday on Friday after fears of an armed suspect was on the school grounds.
Channel 9 news reporter Ben Hennessy told Peter Bell on Radio 6PR initial fears that the man was armed triggered the lockdown.
“There were reports various students may have had to take cover under desks as per protocol in that situation.
“They didn’t know whether the man may have been armed at the time so certainly a scary situation for staff and students at Guildford Grammar this afternoon."
A police spokesman said the man, who was not armed and did not make it onto school grounds, was taken into custody.
Guildford Grammar School principal Stephen Webber said the lockdown was a standard procedure.
"Police were on the grounds of the senior school, and as soon as staff became aware of the situation we initiated a lockdown, which lasted for 20 minutes," he said.
"Following that, we took the roll and then students returned to their desks.
"Even though the man didn't make it on to school grounds and there were no weapons involved, this was just a procedure we follow to ensure the safety of staff and students."
Mr Webber said that immediately following the incident, parents were sent and SMS and an email explaining what had happened.
