An interesting reflection, Annelise. I think you're probably being overly harsh in your assessment of your parenting skills. We can't protect our children from everything they encounter in life. Your sons chose the school they wanted to go to - it could have been a very different scenario if that had not been the case.
I'm a twin too, btw - I couldn't for one moment have entertained the idea of us going to different schools. Our parents gave us the option of going to a private girls school or the local co-ed high school. We chose the latter and to this day, we're glad we did.