When doing my Physics A-level our teacher would often go into more depth and detail on a particular topic then was necessary. When he did this I would switch off, I had a copy of the syllabus so that I knew what (and only) I needed to learn. This meant that I was able to get the required grades but that I was a poorer physicist (and therefore scientist).
This was the same with the other subjects, although as Biology was more my thing I read several Biology-related books.
This is in complete contrast to University, where you are expected to read around the subject to get higher marks (or to pass the exam in the first place). When revising inflammation and other pathology lectures for instance, you would not only go over the lecture slides, but find a pathology textbooks (Robbins, Underwood, Stevens and Lowe) and make notes based on what they had to say.
One of the problems with the current A-level system is that doing extra reading will not get you extra marks on the exam - sticking to the revision guide will. What we need are A-levels that encourage learning for the fun of it. One of my favourite books is "A Devil's Chaplain" by Richard Dawkins. In one chapter he talks about a headmaster called Sanderson, at a school called Oundle.
This was a guy who would leave the school laboratories unlocked so that the students could go and run there own experiments whenever they wanted to. I doubt that this would be allowed ever again.
Although setting an exam with open ended questions in the sciences would be easy to do, thus allowing students who have taken a genuine interest in a subject to excel and show how much they know and love what there studying