mmzz, my DS1 moved from his comprehensive to a super selective school's sixth form that took 100 'externals' on top of the 100+ 'internals.' He's also fairly introverted and not at all sporty which excluded him from the 'beautiful people' groups in his school. I was very worried that he'd struggle socially because he had at school, but his new sixth form was full of eccentrics with whom he fitted in very well. He made some great friends, but it took a few weeks for him to settle. He found that most of his friends were other 'externals' plus a few 'internals' that had themselves struggled socially.
The only downside has been that the school takes from a very wide catchment, up to 30/40 miles and many of his friends live far away. This has meant socialising outside school took rather more organisation than some of them were capable of, and now he's at university, meeting up in the holidays is still a bit hit and miss.
Moving at 16 and making new friends stood him in good stead for the rather greater move to university, though.
The move was something that DS1 himself wanted, though. It was me that took some persuading.