Officers found five decomposing heads in a sack outside the school, along with handwritten notes making threats towards local governors and drug barons. The remains were left in a white bag on top of a wooden box in the full view of school pupils and passers-by.
They were removed by soldiers and police officers, and the area cordoned off. Five decapitated bodies were found elsewhere in the city earlier this week; three of them discovered badly burned in a pick-up truck, and two of them found outside the vehicle.
The BBC report that it is presently 'unclear' as to whether the gruesome find was linked to an extortion threat against school teaching staff.
Dozens of schools in Mexico have been closed since August after teachers took strike action over security concerns.
Guerrero state governor Angel Aguirre has promised more police patrols as well as the installation of security cameras and panic buttons in schools.