I would argue that the largest lesson that can be learned from Avi's Nothing But the Truth is a moral one: that of the importance of telling, as the title suggests, the absolute and most complete truth.
In the novel, we see the staggering consequences that occur simply because Philip chooses to tell a partial truth—one that does not fully represent the facts of what occurred. After being asked on three different occasions to stop humming the National Anthem by Mrs. Narwin, who is only following school policy on the matter, Philip is asked to leave the classroom and is suspended from school. However, rather than come clean to his parents about the reasoning behind the suspension (his blatant disrespect toward Mrs. Narwin and his violation of a standard to which all students are held), Philip claims that he was suspended for singing the National Anthem.
This change in wording may not seem like a big deal, but the way in which it completely alters the context of what transpired serves as the catalyst for a lot of subsequent problems. By claiming that he was singing rather than lying and by painting his suspension as an affront to his patriotic values rather than a disciplinary response to his attitude problems, Philip tells a partial truth. It is that partial truth which results in the school losing critical funding, Mrs. Narwin being forced to "take a break" and then retire early, and Philip losing the trust and respect of all his classmates.
Had Philip just told the whole and complete truth, or "nothing but the truth," he would not have been thrust into the national spotlight and forced to move to a different school. His lie effectively ended someone's career and hurt many more educators and students who should not have been held responsible for his behaviors.
Many lessons can be learned from Avi's Nothing But the Truth, as it examines several issues of morality and civic duty. I believe one of the strongest lessons is to stand up for what you believe in. Though Philip at first may not have been singing because of his civic duty, throughout the entire situation, he never really takes a stand either way. Many of Philip's and other people's troubles could have been lessened if he would have taken a stand. Avi shows through this novel that taking a stand is not only the right thing to do, but it can also be beneficial.