It all depends on what you mean by confidence and what drives it. I have always been seen as confident, almost from birth, merely because I was born thinking I had a right to be heard and made sure I was. It never occurred to me to do otherwise.
I never realised I was seen as confident until I was graduating from university and a group of us were talking about how we felt when we first went to uiversity . Several people said how unsure and uncertain they felt and I heartily agreed, at which they turned on me and said, but you were always brimming with confidence, which was news to me.
I then realised that confidence is a confidence trick, as long as you sound confident, it doesn't matter how unconfident you are underneath.
It has little or nothing to do with where you went to school, I was state educated at a girls convent school.