Ren goes by he most of the time because it's easier. It's not right but it works for now, like a sweater that's a bit too small, but they've stopped growing anyway and the color's nice. Nora mixes up random pronouns, or just says you or calls them by their name. Ren.
He is easier because he, unlike they, doesn't get corrected as a grammatical error by well meaning teachers. He gives ideas that they almost match, if you ignore the pink in their hair and the occasional skirt.
It makes them feel better, less like they're wearing somebody else's skin. They start correcting people who use the wrong pronouns and the wrong words (they're not a boy, not ever, not a boy, at least not that) in senior year because being correct is important, and they want to be out to whatever college they end up going to. Because this is who they are and that's important.