Insurance companies tend to give about 65% off the fire premium. This saving along may take a school 15 to 20 years to break even. But that's assuming that insurance premiums are the only saving, and of course they are not. Sprinklers also allow architects greater design freedoms, for example larger compartment sizes - and there are all the other savings mentioned before.
One difficulty however is that most of the UK's schools are not insured on a stand alone insurance policy, they are 1 of X hundred schools in a local authority. This makes it much more difficult. The largest insurer of these schools typically will pay out for zero pounds up for fire damage to schools that are fully sprinklered, given the deductibles (or excesses) can be in the region of 100K to 1 Million, this is significant.
To persuade someone that sprinkles are needed, all the savings need to be considered, the greater architectural freedoms, the insurance premium savings, the 3 years of interuption it will take to build the new school, the human cost to pupils, parents, teachers and other users of the school.....I could write a long list here, but don't want to repeat myself too much!