The school are being very silly here, to have not followed correct reporting procedure.
Technically it could also be a Health and Safety issue - if you wanted to get really arsey with them about it.
Re Safeguarding, I know a little about this due to my work, and having recently been on a 2 day course. It is my understanding that the new SG procedures are intended to influence any Ofsted grading, being that the school has to prove that not only are the staff trained but that they also implement the procedures effectively. Without doing this, no learning provider can hope to gain higher than a 'Satisfactory' grading from Ofsted. (that is my understanding btw, I'm no expert!)
IMO, they have not provided your son with a safe, secure environment to learn - in dismissing the incident as an 'accident' they have brushed it under the carpet. She may not have intended to cause harm, but she did intend to set fire to a highly flammable aerosol, knowing full well the potential consequence.
YANBU, police involvement is definitely the way forward - even though it will result in a caution, it will hopefully be enough of a shock to her to realise that actions have consequences.
I really hope your son recovers from this and I really feel for him and you.