it depends - sit a scientist down to do a politics or history essay and they'll probably do a lot worse than someone who will fail a chemistry or physics exam. I got an E in chemistry AS but I got A*AA at the end of my A2s, in politics, philosophy and english language (I did also get a C in history - I got all As in AS and happened to do badly in one exam which happened to count for a lot of my overall mark, sadly, while getting a good A2 coursework grade which only counted for something like 25% - ridiculous)
it also depends on the quality of the teachers. I had a dreadful chemistry teacher who would NEVER help me (or anybody) when I was stuck. but my other teachers were *so* helpful it was unbelievable. they were totally instrumental to my success. my chemistry teacher was instrumental to my failure.