She seemed to be “brought in” for the Second Task.
Fleur’s sister didn’t seem to arrive at Hogwarts with the rest of the students - we only see her at the Second Task, when she’s held as the hostage for Fleur. The other three hostages were already at Hogwarts since they were all Hogwarts students, but it seems like Fleur’s sister was brought in specifically for the task, perhaps since there was no one at Hogwarts she cared about much.
“A choir of merpeople were singing in the middle, calling the champions towards them, and behind them rose a crude sort of statue; a gigantic merperson hewn from a boulder. Four people were bound tightly to the tail of the stone merperson.
Ron was tied between Hermione and Cho Chang. There was also a girl who looked no older than eight, whose clouds of silvery hair made Harry feel sure that she was Fleur Delacour’s sister. All four of them appeared to be in a very deep sleep. Their heads were lolling onto their shoulders, and fine streams of bubbles kept issuing from their mouths.”
- Harry Potter and the Goblet of Fire, Chapter 26 (The Second Task)
After the Second Task was over, we don’t see Fleur’s sister again until the champions’ families are invited to Hogwarts to watch the final task.
“On the other side of the room, Fleur was jabbering away in French to her mother. Fleur’s little sister, Gabrielle, was holding her mother’s hand. She waved at Harry, who waved back.”
- Harry Potter and the Goblet of Fire, Chapter 31 (The Third Task)
After having saved her, Harry would be more likely (though not guaranteed) to notice her presence at Hogwarts since he “knew” her - however, she’s not mentioned as being at Hogwarts until the families come for the final task.
In the movie, though, she came in with the Beauxbatons students.
In the movie version of Harry Potter and the Goblet of Fire, Gabrielle comes in with the students, and is doing flips.
She’s also shown in the Beauxbatons uniform, which may imply that the movie intended her to be a student.
However, in the movie, there don’t seem to be any other Beauxbatons students as young as her, so if she is supposed to be a student, that would make her the youngest one brought to Hogwarts by far.
As Valorum suggests in the comments, it’s possible they suspected Fleur would be chosen as champion, and brought her sister specifically in preparation for the Second Task.