The Grange school in Christchurch initiated a short lockdown protocol on Thursday afternoon.
In an email sent to parents just after 4pm, and published by the Echo, the school said students are being kept in the building until it was safe to leave.
A further email sent less than an hour later said Dorset Police had carried out an inspection and everyone was free to leave, adding that all students were safe and well.
Dorset Police tells Bournemouth One they received a call at 3.53pm of a man reportedly seen in possession of a knife in the area of the Somerford Estate in Christchurch.
In a statement, they continue: “There were no reports of any injuries or any threats being made and officers were deployed to the area and carried out extensive searches.
“Following detailed enquiries at the scene, we believe the initial report to have been a false alarm with good intent and there is nothing to indicate any threat to the local community.
“During the deployment, officers conducted a search of The Grange School site as a precaution.
“A number of students and staff who were still on the site were asked to remain in place as these searches were carried out and they were subsequently escorted safely from the premises.”