I actually saw it differently. This movie was made at a time of such outward homophobia. Calling someone gay was an insult to ostracize her. Just because someone starts rumors does not make it true. That was a reoccurring theme about the movie. With that same logic, she was more likely asexual. Because she didn’t show any interest to anyone sexually throughout the movie (except for accusations), so I think that assuming she is plays into the bully rumors. She did not act like a typical girl so they made assumptions that she must be gay. When the Lindsey’s character accuses her, it is a way of others trying to define her rather than it being a revelation of her sexuality.
I loved her character because she refused to let people make her someone she hasn’t. She was young and maybe you are right about her being a lesbian. However, out of reverence for the movie, I’d rather not take the mean girls word for it.