My thesis statement for "A Separate Peace" is this: "The relationship between Gene and Phineas drastically changes as the story progresses." The girl who corrected my paper said this is not a thesis statement, but I think it is. Who is correct?
I as an certified English teacher say that you are.
A thesis is a statement of a proposed fact, that can be supported by evidence.
Your statement is that the relationship drastically changes.
The biggest problem is the word drastically. As drastically is a subjective term it may be difficult to "prove" that the relationship changes drastically.
In the story it should be possible to show what the relationship was at the of of the story. Then contrast the relationship at the beginning with the relationship at the end of the story.
That the relationship has changed can be easily supported. Next provide the reader with examples of changes. Define a drastic change. Then compare the the changes in the story with your definition of a drastic change.
Not only do you have a thesis, it should be possible to prove your thesis.