The 62-year-old operator said he had not encountered such aggression before when flying the drone near gulls.
One of the drone’s propellers was damaged in the incident, at 12:20pm on 8 July.
The UK Department for Transport’s air accidents investigation branch (AAIB) reported today the drone was being used to survey the roof of the school.
An assessment had been made of potential bird interference.
The AAIB report stated: “The pilot launched the drone and flew it manually before engaging the autonomous flight mode.
"The unmanned aircraft system (UAS) flew out on the first track before returning on the second, when a gull flew over the pilot from behind and attacked it.
"The pilot tried to engage manual control, but the front right propeller was damaged in the attack and the UAS, which weighed 6.14kg, fell onto the roof.
"The pilot had previously operated the UAS in coastal and onshore locations where gulls were present, and had been cautious while operating in their areas, especially in the nesting season.
"He had not experienced this level of aggression before and noted that it was one of the black-headed gulls in the area which had attacked the UAS.”
The AAIB classed the damage to the two-year-old DJI Matrice M200 drone as “substantial”.
The drone operator, who reported the incident to the AAIB, had clocked up 95 hours of flying experience prior to the incident at the school.