Reading some of that makes me think of my school.... back in year 10, a few of us took IGCSE maths early. And so in year 11, we had hoped to make a start on AS maths. However, the stubborn maths dept was like: here, do IGCSE further pure maths! IGCSE futher pure maths is pretty much an overlap of the core maths in A-level, and it didn't help that for most of the year our maths teacher didn't know enough math himself. About 4 months before exams, a new teacher came and my results improved somewhat... getting a B overall.
The next year for AS maths, I decided to take C3 module early. But she wouldn't give textbooks and said I shouldn't do it because there wouldn't be any teachers to help because of timetabling problems. When it got closer to exam times, the head of maths, sixth-form and another teacher were telling me I shouldn't do it because I would get bad grades and totally mess up my grades. Still did it and got an 85/100 on my own.
Problem was those teachers all left this year so no one to really prove to about my C3 result.