The central theme of Nothing But the Truth is, as indicated in the title, truth and the destructive power of its opposite, lies. By spreading cruel and unfounded lies about a good teacher, Miss Narwin, who rightfully does not give him a good grade on an important assignment which he did not approach seriously, Philip Malloy does irreparable damage both to his teacher and to himself. When his parents hear his outrageous and completely untrue allegations, they believe him, and take his concerns to the school board and the press. These entities are more than willing to further exploit these lies for their own purposes, reinforcing Philip's decision to take the moral low road, and resulting in a chain of events that ruin both the teacher's career and Philip's own aspirations of Olympic glory.
A second theme that is important in the book is responsibility, as it relates to both the student and the teacher. Philip refuses to take responsibility for his mistakes. He essentially expects something for nothing, and is angry because his flippant performance on an assignment earned him a grade he did not like. Instead of taking responsibility and trying to do better, he responds in exactly the opposite manner, attacking his teacher with deeply damaging lies. Miss Narwin, on her part, has responsibility for her students' well-being, but, despite perceptions to the contrary, little actual authority to hold them accountable. She is responsible for enforcing rules which are handed down to her by the administration and earning the students' cooperation in her classroom, but when all is said and done, the power to make her students toe the line does not really lie with her.