Pupils in north-west London were put on lockdown for more than three hours after police were called to the school at just before home time.
Parents were left waiting for their children outside the school gates for hours as students were reportedly searched before being allowed to leave the school in Pinner.
Several police cars were seen parked in the middle of the road outside the school on Wednesday afternoon and pupils were kept inside until after 7pm – long past the normal going home time of 3.30pm.
One parent told the Standard that “all kids are being searched” and his wife had been waiting for their daughter for three and a half hours.
Another father of a girl at the school said that pupils were being let out “in dribs and drabs”.
He added that the school sent an email at 4pm to say the police were on site and children were being delayed from leaving the school.
At just after 7.45pm, one mother who wished to remain anonymous said: “They [the children] have only just been let out after being searched, each and every one of them”.
A spokesman for the Met Police confirmed officers were called the school because of a police investigation.
Police said they were called on Wednesday but refused to say what the nature of the report was.
The spokesman added: "Officers attended the location. There has been no arrests at this stage.
"Enquiries continue to establish the full circumstances."