I have never been to Spain.
But I do know a number of Spanards quite well.
I can say that, inspite of it's background, Franco, Inquisision and so on, the people there, the ordinary people, are some of the most open and intelegent people you'll ever meet.
If you put yourself in the shoes of a prospective host family, what your teacher referrs to, your transgender, may give cause for thought. That's understandable.
But on the plus side, when a suitable host family is found, you can be sure they will be very accommodating.
Can you ask your teacher if it will be possible for you to write directly to prospective families, explaining your situation and asking them to accept you?