Your teacher wants you to read "great books." The definition of a "great book," for a 10th grader, is: whatever your local school board specifies as a "great book." When I was in 10th grade, we got stuck with "Silas Marner." If you didn't have to read that "great book," consider yourself lucky. After you graduate, you can define your own.
That said, I mostly agree with plawrence. "The Godfather" novel reads like a first draft in many places. But it's a fine example of storytelling, is richly entertaining, and develops its characters very well. It's not a "great book," but it's great entertainment. There's a very honored place in this world for great entertainment.