I went to an all-girls school, so perhaps I am more familiar with this - having letters addressed to, "Dear Alumnae", while my college (which is coed) sends me letters addressed to "Dear Alumni", but really it is such an easy thing. And I get it that there are tons of words in this book and that was the only one I noticed that was wrong. And I have heard authors talk about stuff that they had one way that got fixed in one of the edits. I am sure copyeditors have similar horror stories, I just haven't had the opportunity to hang out with them.
But if your book is about a bunch of girls who attended an exclusive all-girls school then surely they would refer to themselves as alumnae and not alumni. Sure, since alumni is often used for a mixed group in the way that many masculine nouns are, it is not completely incorrect. But, really, when the group is entirely feminine, alumnae is most appropriate. Unless that's the big secret that they've been hinting at with one of the graduates. Somehow, I think not though.