I completely agree with Chisel that there is a formula to most of it and not only is it available by listening to the teachers but there is a wealth of information online including YouTube that will lay it out for you too. There are also past papers, mark schemes and examiners's reports that detail exactly what they want.
I think for maths it is about practise, work out what you struggle with/don't know, work on that and again a wealth of information online walking you through how to tackle it. But don't leave that until year 11, start on that as soon as you can. For science there are key words they want you to use, History there is a set way of answering the questions.
For English there are teachers online who will explain what the mark scheme means when it lays out what a level 6 should contain. I got DC from a bottom grade 5 to a top grade 7 in a couple of months just by going through mark schemes with him for the paper he just sat.
There has to be a general want of learning, applying themselves and understanding that the higher grades you get at GCSE usually means higher grades at A level which then give you choice for universities, so all are open to you rather than only these ones because your A level grades aren't good enough.