teacher definitely the latter. However I would also include schools with larger numbers of high achievers who neglect the needs of the lowest achievers. And schools that are good at progressing lower achievers whilst doing nothing for the more able.
I don't think overall stats and ofsted are what make a good school or a substandard one. If four schools have identical average intakes, one could focus solely on the group slightly below pass, one could focus on just the above average, one on just the below, and another could be doing a fantastic job of providing for every child.
In terms of average numbers reaching a pass, the former 3 would look better, even though they'd only achieved it at the cost of other pupils. Average added value for all 4 would probably be similar too, except when you broke it down individually the first 3 would just be adding a lot of value to some pupils and nothing to others, only the fourth school would have value added across the board.
I'd call all the first three substandard. They might be great if your dc fits into the priority group at the school, but only the fourth school is a good one.
With regards to turning round bad schools, it also doesn't help that the 'bad' ones have to be run into the ground first. The local dire school has got away with it for years, their average 'a-c or equivalent average per child' actually tends to come around national average. When you break it down, you can see they are nearly all c's, and mostly comprised of 'equivalent' not core. Not to mention they screw over everyone above or below that level. With the new targets, sooner or later it is going to get slammed, but when it does, it will be almost impossible to put right. It's already reaching a point where recruitment is a major issue, because although the school still attracts good teachers, even the most dedicated don't stick around. And as you say, which decent head would take on the career killer of 18 months to turn that round with just a handful of decent staff? Whereas if instead they'd been threatened with special measures for massaging pass rates years ago, instead of patted on the back for ticking boxes, it could have been done relatively simply.