umm, there is zero way to read her as not transphobic. She didn't say there might be a problem she said Holly was a boy. Being stuck in your ways isn't a valid excuse for phobic behavior.
Mmm... to be completely clear, I agree that the overall reaction is definitely transphobic. That's obvious. The specifics of this situation, however, create enough doubt for me to be unsure as to whether she's deliberately denying Holly's gender, or just snapping at a student doing something (she believes is) dangerous without really thinking about the implications of what she's saying.
Once again, the results of G-P's actions are 100% transphobic, no argument there. What I'm unsure on is whether the motivation behind them is transphobia- as in, denying Holly's gender- or just assuming that the 'boys cannot approach unicorns' thing is based on physical sex, and then conflating gender/sex in a hasty and angry response to what she saw as a student ignoring her instructions. It's the whole 'do not assume malice when ignorance serves as an explanation' thing; I can honestly see her initial reaction being the result of prior assumptions about how unicorns behave and trying to explain that without really having the vocabulary or familiarity with trans people to know how (since, as has been previously speculated, trans gender identity doesn't seem to be a known thing in the magical world). Add in how it seems to be a very hasty 'what do you think you're doing' snap response to a student (in her mind) disobeying instructions, and thus something that she likely wouldn't have the time to be actively considering her words over, and it's plausible to me that she wasn't intentionally denying Holly's gender (even if that was the actual result of what she was doing and saying).
Her follow-up reaction, meanwhile...
And the fact that she started sulking when proven wrong instead of being happy to learn something new.
That's something that's easily explained by simple pettiness and dislike of having someone less qualified prove her wrong. It shows up all the time in academics and the workforce, no prejudices needed.
None of this paints G-P in a good light, and once again: I do agree that the way she responded was blatantly transphobic. What I'm not certain on was whether the motivation
behind that response was knowingly and intentionally transphobic, or the result of mistaken assumptions and being stubborn about reviewing them.
...also, I realise I'm probably not explaining myself very well, and apologise for that.