I would say that the
only advantage of going to a crappily (lol is that a word?) ranked school is the less competition. You don't know how many of my comprehensive student friends are like, I'm so glad I go to school X and not your school, because of the lower competition. The only disadvantage I see within a comprehensive school is the resources.
I base my assumption on that any successfully academic - or wanting to be, student, should be self-motivated. If you aren't, that means you have crappy time management which means you're not going to go far, selective school or not