I don’t think we know enough about your niece’s situation – e.g., why the guardianship, psychological state, legal issues etc. – to offer you specific advice, though I will say that the posters here have offered very good advice.
Some general advice. I have seen my job as a parent and would see my job as a guardian as helping prepare my children to lead fulfilling, successful adult lives. As such, I have taken the long view. There is no reason a HS junior should know what she wants to do for a career – in fact, as I tell my kids, the two jobs I have had for the longest periods of my adult career didn’t exist when I was in grad school, let alone in college. I think that the evolution of jobs is faster now than it was then, so the lesson is to learn skills and ways of thinking that they can continue to apply as the world changes and that college should be about learning think (and communicate) clearly and how to learn on one’s own.
Assuming you share that goal for your niece, I think going slow and making sure she is engaged and dealing well with the world is more important than pushing things through. @MaineLonghorn echoes something I have told my kids: Life is not a race.
My son took a gap year before college. He needed a surgery but didn’t actually apply to college until his gap year. People told us that would hurt him, but I didn’t believe them and they were, I think, dead wrong. He had more time to study for the standardized tests (see below) and more time to do his college apps as he was not competing with HS courses. He did a number of interesting things that year and so they also helped his college application. He got into very good schools and is doing well – he completed an MBA and an MS and started his second company last year while he was in grad school with a partner he chose from his MBA program and they have been recognized in national publications. He told me that he appreciated the additional maturity he had when he was a freshman and sophomore.
I encouraged my daughter to take a gap year, but she was so excited about the school she got into that she didn’t want to defer, but she then transferred at the end of the first semester. [She’s done very well as well, so it didn’t hurt her that much, but I think her decision-making might have been better with another year].
Finally, I learned on CC about something called the Xiggi method for preparing for standardized tests from posts by a guy on here named @xiggi. I’d look it up, but it largely involved taking all the real, available practice tests. It worked well for both kids. My son spent half a day each day for three weeks and then took the tests.