Make the most of the opportunities that you're given – and no, I'm not just talking about academics. Selective entry schools provide many extra-curricular opportunities that allow you to develop yourselves as people, and being able to pursue such opportunities represents, in my mind, one of the biggest strengths of selective entry schools.
Put your hand up for everything you think you can reasonably manage and try things you’ve never thought about before because you never know if you love something until you give it a go!