I don't just mean that in class she is around top-performing kids. I mean that that's what she lives and breathes. Honestly sometimes I find it a bit unnerving, but also it does definitely contribute to academic success. Every day she is around kids who either a) have an incredible work ethic or b) are extremely intelligent or c) both. There's selective trips. There's prizes and muse boards dedicated to kids who have done well. They get handed out photocopies of stellar work from children in their year as an incentive. I know it sounds awful typing it, but it's actually a wonderful motivator for her. And it's not just focused on things like Maths and Science- there's lots of these selective trips and honourable mentions about sport, art, music etc.
I remember once I said, "it's okay to just get good results in your GCSE, you know. They don't have to be fantastic." I do worry that it pressures her!
DD replied, "oh, I know. It wouldn't be the end of the world to get 7's in one or two." 
I'm not sure if it's just DD's school that's so highly competitive or all grammar schools. I doubt it would work for all kids but the atmosphere of competition, the work ethic, the aspiration coming from all corners undoubtedly motivates DD. I think in comprehensives, even in top sets, you're coming into contact with kids who just don't care and the general atmosphere of schools is affected by that. While I don't doubt that DD could do excellently in a comprehensive, it's made a massive difference in how much work she puts in (she's always been very intelligent, but could be a bit lazy).