A primary school pupil threatened a teacher with a knife before the weapon was wrestled off him by another member of staff.
The boy, aged under 10, confronted the woman teacher after a row at Darell Primary and Nursery in Richmond, South West London, The Sun reports.
He had taken the knife from a kitchen area and stormed into the staff room before he was restrained. The boy has since been excluded.
One shocked mum said: "We should have been told by the school - and told their measures for preventing something like this."
The school, rated good by Ofsted, did not call police as the boy is under the age of criminal responsibility.
Headteacher Laura Whateley confirmed the incident but said no one had been 'at risk of harm'.
The incident happened just three days before teacher Ann Maguire, 61, was stabbed to death in a Leeds school last month.
Her funeral is being held today (Friday). A 15-year-old boy is charged with her murder.