Police were called to a St Helens primary after a member of the public spotted a man acting suspiciously in a field behind the school.
Officers were contacted at 2.20pm today (Monday) to reports a man had been seen on a public field at the rear of Merton Bank primary school.
The man was challenged by a member of the public and left the area.
As he did it is believed he discarded a small knife.
An extensive search of the area was carried out.
The man is described as white, in his 50s, wearing a white T-shirt, black shorts and wearing a cap.
Superintendent Louise Harrison said: “None of the children or staff at the school were approached or spoken to by the man but a member of the public raised concern about this man, who was on a public field at the rear of the school.
“An extensive search of the area has been carried out and we have spoken to staff at the school. At this stage the incident is being treated as suspicious activity.
“Neighbourhood officers regularly visit schools to work alongside teachers in giving children appropriate stranger danger advice.
“I would also like to reassure parents in the local community that we take all incidents of this nature extremely seriously, although they are thankfully rare.
“I would ask parents to continue to be vigilant and report any concerns they have over suspicious circumstances.
“I would also encourage anyone with further information to contact their local neighbourhood officers, call 101, or Crimestoppers anonymously on 0800 555 111.”