While still at school it is useful to do well in physics and maths, or at the very least maths. For becoming a pilot the grades is not really important. There are various options while you are still young. Apply to join the RAF and they pay for the training. Airlines do sponsor when they need people and there are a lot of threads on pprune.org which endlessly discuss who is recruiting/sponsoring etc. Try to get a trial lesson at a local flying club to see if you like it, and be sure that you have no medical conditions which preclude flying. Is it worthwhile? Oh yes, there is no job quite like it. I entered it rather late in life, but fly professionally in a freelance capacity. Best of luck.