I am horrified.
The school is clearly completely ignorant about this disease, and they need to develop a well-informed protocol immediately. The teachers and staff should be aware of what an attack looks like, and should be able to see if the blue inhaler has been used a few times to no effect that an ambulance is needed.
Could you have your GP source some material for you to send to the school governors so that a protocol could be developed? Could he or she also write a strongly worded letter to back you up when you send your own strongly worded letter to them?
Your DD should be allowed to keep her phone on her at all times, even if there is a no phones rule in place. I agree with Tinkly - the rules go out the window, and even a friend could use DD's phone to call you if the asthma leads to confusion or such intense focus on breathing that DD can't think. But this should not be the job of DD or her friends. The staff dropped the ball in spectacular fashion here.
I lost a sister to this disease. All asthma deaths are preventable, but there is a fine line between the place where medical intervention will work and beyond which it won't. There is no way a student should be allowed to approach this line in a school, where there are supposed to be protocols in place.
I would send a letter to the HT ahead of your visit to the head of year, and CC the head of year. Pay the extra to request a signature on receipt from the HT, just to put the wind up her. CC the board of governors too, if there is one, and I would copy OFSTED too.
I would spell out A - what happened, and B - what should have happened. It would be handy to have a letter from the GP or hospital to back you up.
Best wishes to your DD. Hope she recovers and won't be too badly affected by this (a bad attack can leave you feeling stressed and worried about the next attack), and I hope her friend is ok too.