Your schedule, to me, sounds fine. It's recommended that for every hour you do in class, you do an hour at home so about 3-4 hours per subject per week (which sounds roughly like what you're doing now). As for your parents, I know it's easy to say, but they're your exams. Don't let other people be telling you what you should and shouldn't be doing. You know how much work you need to do. Personally, I can spend 4 hours on one subject but only need 1 for another to learn the same amount of stuff. You might need to explain to them how much work you're doing and how your grades are reflecting that (you must be bright to be aiming for all A*s). I would maybe suggest getting them to test you on some things each night so they can see that it's fine as it is. Even better, those extra 3 hours that they recommend you do? Make that 3 hours worth of time for them to test you. If they can't afford that time, what makes them think that you can?