You should talk to the school where you plan to enroll you niece first, see what type of visa they require. If the school is not formal, for example, someone teaching english at his home, or at some church, you may not need a student visa at all.
But either way, it is going to be very hard for your niece to get a visa to come here. And if she is coming by herself, you probably need to establish a temporary guardianship first. Lots of procedures to go through. To me it is not even worth the trouble...