About 40 firefighters tackled a blaze at the derelict Drumoyne primary school on Shieldhall Road in the Govan area of Glasgow which was reported at around 2am today.
It comes after a similar incident at the former Broomloan Road primary school/Govan parish school board site on nearby Broomloan Road on Tuesday.
The last Scottish Fire and Rescue Service (SFRS) appliance left the scene at 8am, meaning firefighters and resources from Glasgow City, Renfrewshire and East Dunbartonshire were required for about six hours.
SFRS group manager Iain Goodlet, head of protection for the City of Glasgow, said tackling the blazes took up significant resources which could have been needed elsewhere.
He said: “The crews and appliances required to tackle needless fires are unavailable to respond to house fires, road traffic collisions or any other emergency that happens during that time.
“This obviously risks causing delays in getting help to an incident where every second is crucial to rescuing a person in immediate danger.
“Anyone responsible for setting fires needs to know there could be truly horrendous consequences and they need to consider whether they could live with themselves if someone was killed.
“It’s also important everyone knows that we work incredibly closely with our colleagues from the police to investigate fires and see that anyone setting them is identified and held to account for their reckless, criminal actions.”
SFRS said they were investigating the fires and would share information with police.