I think upper middle grade is a great term, and really clear what it means. Percy Jackson, Savvy, that sort of thing--something more complex than Ramona Quimby, but not full on, angsty teen. "Tween" means that age group to me, too, but girly--kind of chic lit jr.
Those books do get shelved in different places, though--I've been to some libraries where things I felt were well into YA were down with the middle grade kid books, and the YA books were very old. My current library skews strongly the other way, with books I feel like are definitely middle grade over in in YA. The new children's librarian is changing things, though, and has at least divided the younger and upper middle grade into different areas of the kids' section. It's nice for my kids, who are looking for UMG.
My previous library had a lot more money, so whenever there was a book on that border, they bought two, and stuck one in MG and one in YA. It was a great solution!