I had co-education most of my life , even my religious education, I was taught how to recite the Holy Quran and basic Arabic by a Palestinian women and her daughters. The only apprehension my parents had was perhaps I would grow up too girlish so they had to curate the list of my friends and activities and made sure, there were more male friends and I spent quality time with them instead of girls and no sleep overs at any girls place. But I learnt a lot. So personally I think it should be an options and parents can select.
However at certain age when hormones are raging and energy is overwhelming and kids start to get curious about each other, I would say it is better to separate them or parents and teachers have to keep a stern eye on children's activity. Quite a lot of things happened that made my life a bit stressful which I may not feel comfortable sharing here but I thinking retrospectively, I feel non-co-education environment may have been less stressful as my family is educated but conservative. But since sophomore level I developed my own strong personality and an invisible iron dome that does not let any transgression..