Firefighters and police were called to Asmall Primary School, Tennyson Drive, Ormskirk, at 7.47pm on Thursday June 17 to reports of a fire.
A significant amount of the building has been damaged and work is currently ongoing to ensure it is safe.
Police are now working alongside Lancashire Fire and Rescue Service to establish the cause of the fire, which is currently being treated as arson.
No arrests have been made at this stage and detectives are asking people to check their CCTV or dashcam footage to see if they have captured anyone acting suspiciously in the area before or after the fire took place.
Det Sgt Elizabeth Martin, of South CID, said: “This fire has had devastating impact on the school and the local community as a whole and we are working to establish the exact cause of it.
“We have already made a number of enquiries and we are now asking the public to come forward if they witnessed anything or have any mobile phone, CCTV or dashcam footage which could assist our investigation.
"We are particularly keen to obtain footage from before and after the time smoke started to appear from the building.
"We would also like footage which shows anybody acting suspiciously in the area in the lead up or after the fire was started.”
Anybody with information is asked to email [email protected] or call 101, quoting log 1424 of June 17, 2021.