Police say a man has been spoken to after he entered the grounds of Rainhill High and threatened two pupils.
Officers were alerted to the incident on Thursday, June 7 shortly after 8.40am.
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The school say the man was a parent angry about an incident which occurred outside the grounds of the Warrington Road school.
A force spokesman said no reports of assault or anyone being injured were received.
He added: "The man has been spoken to by the Schools Officer and given advice on future conduct."
Rainhill High say teaching staff intervened to prevent the man gaining access to the school building.
It is unclear what sparked the incident but it believed to relate to bullying.
A school spokesman said: "A parent did come into the entrance to the school looking to confront someone else’s child about an incident that had occurred outside of school.
"Thanks to the prompt action of staff, no children were hurt and the parent did not gain access to the building."
The school has had issues with bullying over recent years.
And earlier this year a parent posted a recording on social media of pupils threatening her daughter.