I really hate the idea of social engineering on this level. I'd much prefer to go back to grammar schools and the eleven plus, though that had its flaws of course. I hate the idea that high-achieving poor children might fall by the wayside in state education, because their parents can't afford a private school (and thus secure superior teaching and opportunities). I can see why people think that messing around with the grades is the answer, but it really isn't. As do that, why not simply bump up the grades of all state students... you get an extra 10% added to your mark if your state school is average, an extra 20% if it's struggling, an extra 30% if it's in a really bad area etc. No-one would think that was fair, but it's essential the same thing. It's just a sticking plaster on all that's wrong with our education system.
Disclaimer: After being an overachiever at my tiny state primary school, I went to an excellent private school, on a government assisted place (we were very poor). I got excellent results at GCSE and A Level. I went to an excellent university after receiving six offers in my final year at school. I then messed around and got a 2.1 degree because I no longer had anyone pushing me to do my best, and I suddenly had freedom from a high-pressure environment. I was a little mortified by my 2.1, and went on to get a distinction at MA then a PhD, before moved into publishing.
My brother, meanwhile, equally intelligent but completely without motivation of any sort, went to state school, messed up his GCSEs and A Levels, couldn't be arsed with university so worked in a factory for several years. He then managed to get an office job, moved into IT, became a consultant and earned about twice as much as I do, working from home and choosing his hours.
What lesson to take from this, I don't know...