The trespasser climbed a fence before entering the grounds of the school in Crabtree Avenue at around 12.50pm on Tuesday (February 22).
Police quickly attended the scene and the intruder - a teenage boy - was detained following a search of the area.
"We were called ... to a report of nuisance," a spokesman for Lancashire Police said.
"He [the boy] was spoken with and is to be returned home."
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