I also wanted to know the difference between middle grade and upper middle grade. So I emailed Mary Kole, an associate agent at Andrea Brown Literary. Her blog: www.kidlit.com has proven itself a great read.
Her response was that as long as their is no boob touching or below the belt stuff it should be fine but to wait to see what an agent says.
Now about Upper Middle Grade vs. Middle Grade: Basically, she said that the main character can even go up to the age of fifteen depending on the type of story and the character's maturity. She said this was because kids like to read characters older than them. So if the main character is 15 most likely the kids reading would be 12-13.
Hopefully, this'll help some of you middle grade writers out there :]