I'm always asked to assist good readers find books beyond their grade level, which can be a difficult task, as you have to take into consideration the maturity level and content matter of books that may or may not be suited for younger readers.
I think the Children's Book Council does a great job coming up with a book list for these students.
Chosen for children who are reading at an advanced level
Here are 75 books chosen by the ALA-CBC (American Library Association & Children’s Book Council) Joint Committee to provide guidance to parents, caregivers, teachers, librarians, booksellers, and anyone interested in discovering books for children who read at an advanced level and are seeking more challenging, but still age-appropriate, books.
Printable handout of the 75 books