I'm going to buck the trend here, and say I don't think it's acceptable to send 11/12 yo kids home, without warning, and without specifically assessing if the kids were able to get in home, etc.
Here, children that age would still be in primary, and the vast majority would go to some form of childcare after school, rather than home alone.
I have a 10.5 year old. Neither he nor any of his friends have mobile phones or keys to their homes. I also know plenty 11/12 year olds who would be the same.
I also think it is wrong to assume that children should know to call to a neighbour. Around here, you'd be hard-pressed to find people home during the day.
We have just started to leave our 10 year old at home alone for short periods of time (10-15 mins). I wouldn't be overly confident that in a years time he would be able to be at home alone unexpectedly for a number of hours.
It was reasonable for a parent to assume that when their child went to school in the morning, that they would be there for the day. A power cut doesn't pose any danger to the kids. I would think it far more appropriate that the school contact parents and tell them of their need to send children home, but to not actually do so, until they had received a communication from the parents that they were agreeable with this. It is not acceptable to just send kids home.