They're not as important as your teachers will try to make you believe, but they're still important and obviously it's in your best interests to do as well as you can. The grades needed to progress to A-levels vary hugely according to the school or college, but chances are, if you're in a top private or grammar school that asks for really high grades, you'll get them because you were intelligent enough to have got a place. As for university, it depends on the university itself and the course; obviously medicine, law or English at Oxbridge will demand higher grades than a less popular course like maths or languages at a lesser university. Generally though, if you get all A*-Cs and an A*/A in the subject you want to study, AS grades, A-level predictions, personal statement, reference and interview if you have one will be much more important.