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The trans reading of that line would be that she was seen as a wizard when she was at school, so they didn't recognize her now.
How would they know the character is trans? >

She has a line of dialog where she says some of her old classmates didn't recognize her. "Took them a second to realize I was actually a witch, not a wizard.”
It could mean she's trans, or it could mean she wears masculine clothes and they didn't notice she's a woman at first glance.  They're leaving it open to interpretation so they don't have to commit one way or the other >

If the character was trans, they wouldn't say they were actually a witch...
Imagine a trans man all done up in men's clothing telling people "Took them a second to realize I'm actually a woman, not a man"