Bottom line is parents aren't always right but they are right way more than 14 year olds are. In extreme cases emancipation is available. But really that is not applicable in this case because this "child" is mere months away from being 18 and able to do whatever she wants. The question I'm left asking is why didn't she just chill for a few months? I mean with all the persecution and suffering in the world today is it really that hard to wait a few months before posting something on twitter defying your parents? Again, parent's and daughter need to learn to communicate and be rational.
This idea of telling her to just chill a few months strikes me as odd, and I'm someone willing to give the school and parents the benefit of a few doubts. Not that I doubt that both parents and school see gayness as sinfulness, but that doesn't mean that they'd necessarily go into so-called conversion therapy. (Not that Andrew likely considers that a distinction worth a difference.) Also, we still don't know the whole story and there could be other factors beyond being gay that led the parents to send her to a boarding school. The only absolutely sure fact is that there was already a lot of ill will between her parents and her aunt.
But back to the point at hand. Teenagers are not usually renown for self-control and frankly, I don't see that this is the sort of thing she should have to hide for even a few months. Even if one were to think it sinful, it isn't harmful to either oneself or others. (At least not harmful in the physical sense that laws in a land like ours with freedom of religion should limit themselves to.) And in the moral sense, the whole history and ethos of Christianity speaks against the validity of forced conversion of any sort, let alone one perceived specific sin. At most one might condition a person against engaging in a particular behavior, but we can still sin in our hearts, even if kept from sinning in our loins. We all sin, but the beauty of the message of Christ's true message is that grace is available to us if we but ask for it. (And as a Universalist, I don't even hold that you necessarily need to ask of it from Christ himself. The Way is narrow and few will find it, but that doesn't mean only One (or Three) ever found it without guidance.)