Sick thugs snapped chickens’ necks after breaking into the Lancaster school where they were kept.
The birds were slaughtered after the offenders gained entry to an area of Ryelands School on Torrisholme Road.
They approached a shed and killed all nine chickens which were inside.
Pupils have been left distraught by the incident, Lancashire Police said, and officers are appealing for witnesses.
Police believe it happened between 5pm on November 8 and 9am on November 9 and released the first details of the incident today.
The dead chickens were found by a member of staff the following morning.
PC Dave Pinnington, of Lancashire Police, said: “The offenders clearly knew where these chickens were kept and I would encourage anyone that may have information about this incident to come forward.
“The children at the school have been left very upset and anybody that could assist with our investigation is urged to contact Lancaster Police on 101.”
Information can also be given to Crimestoppers anonymously on 0800 555 111.