In all honesty I hated school and didn't try very hard. Science was always my favourite subject even through secondary although I still disliked the lessons, I also hated maths and I left school without a GCSE in it.
I didn't have a single A or B in any subject when I left school and spent the first year out of school (aged 16) working with horses. But I turned 19 in January this year and I've just firmed an offer from a RG uni for physics starting this September, the time out of school really allowed me to explore and learn in my own way. Sometimes school just isn't the right environment to allow people to flourish and for some of us you just need to be allowed to go out in the world and walk your own personally made academic path for a while.
I think that there are many contributing factors as to why girls don't go into STEM degree's and the main factor is the school environment, it needs to be changed.