That depends. If the quality of teaching sucks, then you tend be left pretty much on your own to learn. In such cases you can lack a lot of info that is not in the course books and online.
But at the same time, if you end up having no motivation to even attempt to help yourself out and learn on your own, then really, you end up with pretty low grades.
Self teaching due to a crap teacher can still score you a high grade but it's all down to your determination in the end. While you don't have much effect on your teachers/college and their quality of teaching.
It's good remembering that most of the "well known, hard to get into and great" colleges/sixths forms are based on as little guidance by the teacher as possible.
Also, in every college/sixth form you become more independed than you were in school. You're expected to be anyway. Of course, there are plenty of college teachers who are great and will offer as much help as they can in having you achieve the highest grade possible. But even then, it's still down to you to get that grade in the end. All they can do is help and teach you all they possibly can