hence why if you're shit at one of the three you don't choose that subject at a-level, and if you're shit at all three it's probably best to consider going into degree apprenticeships or whatnot. plus, all you need is Cs in maths and english and you're basically set for most things in life.
usually you find an applied version of one of those three better; e.g. i got a C in maths at GCSE but chose computer science for A-level but i find the applied maths of comp sci to be very nice and understandable. i may not be able to do 3D trig but i can do A* search, djikstra, reverse-polish notation, etc etc etc etc
some parents like to have a more disciplinary attitude towards their children if they don't get good grades, but i doubt it really affects the child that much. i have a friend who is getting AAA in all his subjects who has extremely strict parents, and he gets on just fine and isn't stressing, and he doesn't do completely perfect on every essay
nobody should be rewarded for getting Us, Es and Ds, because these are piss poor grades and won't get you anywhere in life, and you should be told you're doing shit and should be improving, and then teachers help the kids out. my law teacher does "tutorials" which are personalized revision sessions that she sets up for a 1 on 1 between you and her to learn a particular part of the subject that you are struggling with, for example
the only major problem i see about attitudes in education is the very negative stigma of BTECs, but even there it's never a good idea to reward someone for doing shit, because they've got no incentive to not do shit