The principal did not say what the incident was.
Nearby dairy, Jervois Foodcentre owner Jay Kaur said she had not been told what had happened but was stopped from driving down the street to work by officers.
"There was a police officer with a gun and two others on the other corners."
She said that time of day was usually a bustling time of trade but no customers came into the store because they were not allowed to drive down the street.
From 2.30pm the police had stayed in the area for about half an hour, Mrs Kaur said, but they left when St John's College students began filing out at the end of school.
Some students had told her that they had heard the dairy had been robbed.
Still none the wiser, she said "they [the police] should have told us what had happened".
In addition to the college and dairy there is also a preschool in the residential area.