The National Education Union and the Fire Brigades Union have written to education secretary Damian Hinds asking why schools - including one at the base of Grenfell Tower - have been classed as being at too "low risk" to be given fire sprinklers.
The letter follows last year's devastating fire at Grenfell Tower, in Kensington, west London, which caused 71 deaths.
The unions accuse the government of "paying lip service" to its own fire safety guidance, and "allowing it to be routinely flouted in the rush to build new schools as cheaply as possible".
They point to government guidance that states all new schools should have sprinklers fitted “except in a few low risk schools”.
However, Selsey Academy in West Sussex, which burned down in 2016 , is being re-built without sprinkler systems fitted, the unions say, adding: "This is not an isolated case."
Kensington Aldridge Academy, based below Grenfell Tower, also has no sprinkler system, the unions state. They say: "We would question how a large multi-storey secondary school in central London could possibly have been assessed as one of 'a few low risk schools'. "
They add: "None of the 35 Croydon school building projects since 2012 were fitted with sprinklers. Thirty-two new schools are due to be constructed in Northamptonshire, without sprinkler-systems fitted, despite the clear 'expectation' that this should happen. It is inconceivable that all these schools are 'low risk'."
In a 2015 Westminster Hall debate, Conservative MP David Amess said that, since 2010, only 35 per cent of new schools had been fitted with sprinklers.
Sprinklers save lives, reduce rebuild costs and time, mean less disruption for children and staff, and reduce risks to firefighters, the unions argue.
Kevin Courtney, joint general secretary of the National Education Union, said: "Grenfell Tower should have been a defining moment in the way we view safety in public buildings but it seems that health and safety is still seen as an opportunity to cut corners and save money.
"The same protections which apply across Wales and Scotland should cover England too. It’s clear that the only way forward is for the ‘expectation’ that sprinklers be fitted to new school buildings to become a legal requirement. Otherwise the policy will continue to be flouted."
Andy Dark, assistant general secretary of the Fire Brigades Union, said: “The government’s attitude toward fire safety is shockingly cavalier. Sprinklers play an important role in preventing the growth of fire, limiting damage to buildings and saving lives.
The cost of fitting sprinklers represents a very low investment when weighed against the potential threat to life, the damage to buildings and the disruption of children’s education if there is a fire in a school. It is essential that the government act immediately to make it a legal requirement for sprinklers to be fitted in all new school buildings.”
The Department for Education has been contacted for comment.