The question really is did she back out because it was overwhelming, or because she wasn't a lesbian or bi in the first place? The story doesn't make the question clear, although the fact that she kept comparing the older girl to a guy ("boyfriend", looks like a guy, husky voice, sounds like a male singer she likes, etc.) suggests the latter.
Even if Miya was straight prior to meeting sensei, I think this experience probably made her at least bi. The reason I'm pretty impressed with this one-shot is precisely because (and I don't mean this in any derogatory way) of Miya's self-awareness of being a 'brat'.
Like, the thing is that a 14 year old really doesn't have a solid idea of everything that exists in the world or all the possibilities of their life. Jumping into a relationship based just on emotions at that point is a pretty near-sighted thing to do, and a relationship with an older person who was supposed to be your teacher and is even a woman could have even more drastic consequences. A young person who is aware of and accepting of their immaturity (even while taking their feelings and experiences seriously) is something which I admire, 'cause I think that's the most responsible way to go about living that stage of life.
As for the emotions Miya felt themselves, however, they were certainly genuine/non-trivial, which is the key to why I think she'd take the possibility of female-female relationships in the future more seriously. You don't forget your first love, and this was certainly an event Miya will hold onto preciously even if she had to give up on it. The main idea is that realistically, 5 years or so from now, we really have no idea whether Miya and sensei would still be a good fit for each other, what kind of people they will be. However, there's no doubt that this event would've been part of what shaped them both, and in a positive way.
For my part, in any case, I feel pretty positive that most of Miya's relationships in the future would be with girls.
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