I think the difference is that the French perceive obvious religious clothing similarly to someone wearing a T-shirt saying "I vote republican/democrat" in huge letters -- though I don't know, maybe that'd be considered fine too in the US (US schools, that is)?

I see it as clearly unnecessary/voluntary (so it's quite different from being gay like in your example), so it's an attempt to advertise, which leaves a bad taste in a place like a school.