Hi.
Not sure about min. grade required, but i am doing this for my A-levels;
I did my GCSE's(O-levels) and i didn't complete the exams, i left half of my exams behind.
I am going to start A-levels this september, and i can give O-level exams slowly, meaning 1-3 exams this november '13, 1-3 exams march '14 and the rest november '14. I can still sit in A-level class and get education, but i cannot give A-level exams until I CLEAR my O-levels. Ask the principal in your school if that is possible
Anyways, nice to see you want to study, keep the good work up and best of luck^_^