I think that you're actually concerned with different things: your concern is with the girl who wants or is pushed to wear a head-scarf to school. The argument about coercing that girl not to wear a head-scarf is concerned with defending against a perceived threat to the status quo - a hard-won and insecure status quo to be sure. Your arguments about that girl's well-being have little weight in that frame.

I don't buy the "defending laicism" frame either: forcing Muslim girls not to wear headscarves in order to keep religion out of schools seems analagous to requiring Christian girls to bare their breasts at school in order to keep religion out of schools. Forcing girls to dress immodestly by their families' standards is not the way forward if you're concerned about their well-being. If your concern is ramming your standards down their throats however ...