I have not read all the posts but PutTheBathOnPlease YANBU.
I would be angry. My dd doesn't have a key, doesn't want one, and doesn't want to be home alone. She is 12. She has some learning difficulties.
Not all kids have mobiles, not all kids with mobiles remember to take them every day or keep them charged up.
I am not sure why the children could not be accommodated at school safely for the normal school day.
There might not be hot dinners but something could be worked out. Buying bread and fillings and letting kids make their own lunch etc.
Torches to go to the loo. Does the school have any?
Wearing their coats in the classroom if cold.
I wonder if without Data projectors and on line facilities the teachers could not manage the lessons.
Who knows, local children's homes may also have been affected by power cut. So kids leave a cold school to go to a cold home.
Secondary goes from age 11, lots of 11 year olds are not great when left alone for hours, unsupervised.
I wonder if the school have learnt anything and would be better able to cope with this if it happens again I seriously doubt it