The incident comes as new figures reveal that the number of weapons being taking into schools is growing.
Data disclosed under a Freedom of Information request by police forces in England and Wales showed thousands of weapons were seized from schools. At least one in five incidents involved knives.
It has been confirmed that earlier this month a year six pupil at English Martyrs Catholic Primary School in Sizer Street, Preston, was in possession of a knife on the premises.
It is understood that the boy had “mistakenly” left the weapon in his jacket pocket and reported the error to staff at the start of the school day.
Headteacher Annalise Howarth said the incident “was dealt with extremely quickly and safely by staff in accordance with the robust procedures we have in place.”
She said it was “inappropriate” to discus the matter further but added: “I would like to reassure parents that no one was threatened or in danger at any time, and that although events like this are extremely rare in our school, they are taken very seriously as the safety and security of our pupils is paramount.”
Lancashire County Council said safety and security were taken “extremely seriously” and incidents were low.
Bob Stott, the county’s director for education, schools and care, said: “We take safety and security extremely seriously, but with more than 630 schools in Lancashire, and a school-age population of around 200,000, it is inevitable that a small number of incidents occur.
“Fortunately the number of incidents is very low, and serious crimes in schools in Lancashire are extremely rare, however, we do take any incident of this type very seriously.
He added: “We’re not complacent, but our most recent pupil survey tells us that they feel safe at school, and anyone who has visited one of our schools recently will know they provide a secure environment where children enjoy learning.”
National Police Chiefs’ Council lead for knife Crime, Chief Constable Alf Hitchcock, a former Lancashire police chief, said: “Schools should be free of weapons and all children should be able to learn without fear or violence. Carrying a weapon of any kind in schools is not an issue for a school to deal with alone; police and partners will always be willing to work with them and take appropriate action.”
Geoff Barton, general secretary of the Association of School and College Leaders, said: Schools work closely with the police to protect and educate their pupils, and in some cases police officers are stationed in schools. Where appropriate, schools conduct searches and use metal detectors, and they implement robust disciplinary procedures against anyone found in possession of a weapon.
The Government has increased teachers’ powers to deal with pupils suspected of bringing banned items into school.