What is the theme of the book nothing but the truth by Avi?

The theme is mostly truth, but there are also other themes. Like Immaturity, manipulation of the facts, Distortion of the truth, Failure to take responsibility, media accountability and bias, local politics (the ends justifies the means), Understanding (or lack of it), and school system hypocrisy. All of these themes are represented throughout the book, although some are represented more strongly.

For example:

Instead of telling Ms. Narwin how he felt about her unfairness and her teaching Philip rebelled. (Immaturity)

When Philip tries to tell his father about school and why he isn't going out for track. (Lack of understanding on Philip's father's part, and failure to take responsibilityon Philip's part for lying about track.)