I've heard 2 things about them:
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They're built in crap areas (like mine) and provide an opportunity for working class children to be properly educated.
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The criteria for getting in are so opaque that only the well-informed middle classes can get their children in and they're even more selective than grammars.
So, what's the real deal? Currently in this area, it's the 11+ and DS either gets into a grammar or we move (seriously, the local sink school is one of the worst in the whole country, not just the county). Will the appearance of an academy be relevant to him or not?