The entrance to Queen’s Drive Primary School in Fulwood was smashed open with a paving stone at around 11.30pm on Monday (March 29), with thieves climbing through a shattered window before rummaging through the hallways and classrooms.
The school's security alarm alerted police to the break-in, but the intruders had fled the scene before officers arrived.
Lancashire Police's neighbourhood policing team said no arrests have been made at this stage.
Headteacher Julie Webster said: "There was a break-in at the school at around 11.30pm on Monday, to which police responded as a result of the security alarm being triggered.
"A window was broken in order to gain entry, and some items of small value have been stolen, however fortunately nothing of great significance.
"I have been at the school to ensure the building is secure and help with the clean-up, and the police are investigating."
Anyone with information is asked to call police on 101.