What is the grammatical difference between a title of a story called:
A girl who wanted to be a boy
and
The girl who wanted to be a boy.
The story is about a girl who discovers she is in the wrong body, and wants to be a boy. Her temper causes an incident, which leaves her brother in a coma. Now she finds herself in the middle of a police investigation, her parents split up, the older boy who is responsible for her brother ending up in a coma, blackmails her – and that is just the start of her problems.
Google books shows both versions:
Best Answer
As noted in Dan's comment (now deleted), it depends on whether that particular girl's story is the focus of the work, or whether the work is more generally about girls who want to be boys (albeit, presumably, while relating what happens to the titular girl).
Compare the books The Boy in the Stripped Pyjamas or The Hundred-Year-Old Man Who Climbed Out the Window and Disappeared which are both about very specific characters, with the play A Woman of No Importance which -- while it contains "the woman of no importance" (Mrs. Arbuthnot) -- is not primarily about her: it is primarily a satire on the English upper-class of the time.
From your synopsis:
it feels like the story is more about the girl and the developing situation she finds herself in than it is directly about being in the wrong body (even if -- I'm guessing -- the chain of events is initially sparked by her anger at her feelings not being taken seriously).
In that case, I believe "The Girl Who Wanted to be a Boy" would be more fitting.