This is an example of a hard reality: the affirming mainline denominations including the Episcopal Church do have denominational affirming policies, but there are MANY local parishes that are not affirming. The Episcopal Church didn't resolve gay marriage until 2015 when the convention voted to allow marriage ceremonies for same gender couples. Until then, there was only a special blessing ceremony allowed-- not the same as a wedding ceremony. I'm guessing that as a private school in Florida, the Episcopal school has lots of students from religiously conservative homes. And the administrators were afraid of the stink these parents might raise. Or its possible the sponsoring parish, if there is one, is one of the many parishes that do not do same-gender wedding ceremonies. Much work to be done.