I'd tell your niece not to worry about a thing. Chances are, she might get a lot of homework, but I can't imagine she won't be able to handle it. A lot depends on the school she goes to in America, of course, but I'm pretty sure the work she'll be doing there will be no more difficult (if not easier) than the work she's used to doing in Europe (which is where I assume she goes to school now). Besides, school in America can be a lot of fun, especially if your niece is into sports or cheerleading, for example. I'm sure she'll have an unforgettable, life-changing experience!