I did higher maths after int GCSE in yr 9 and 10. I thought I'd coast it but mate, it is hands down THE MOST DIFFICULT paper of every GCSE. I'm not trying to put you off but even being as good as I was, of our set being top set, only 8 people were selected of which the potential A* being 5 of us candidates had an extra class friday afternoons.
I hope that gives you an idea, I got a C in my GCSE exam in yr 9