The phrase "grade fiction" refers to books or stories that are written for a specific age or reading level, usually for students in a particular grade at school. Full definition
I've written a middle grade fiction story that runs 9,000 words, which seems to be no man's land for fiction. (nybookeditors.com)
We represent more than 50 authors who write romance, women's fiction, science fiction / fantasy, thrillers, young adult and middle grade fiction as well as select nonfiction titles. (threeseasagency.com)
On the children's side, her clients are primarily young adult and middle grade fiction writers with a very select number of picture book projects. (dclagency.com)