The great thing about Nothing But the Truth is all the different points of view you are given. The author uses Philip's journals as well as how others react to him to give the reader an idea of who he really is.
From Philip's journal you can gather certain facts. Philip loves track. Philip does not like English, he seems to hate it. He thinks his English teacher does not understand him. He thinks highly of himself. He thinks he's funny and fast. He is a good student in some subjects, such as math.
However, when we see others react to Philip we get a different idea of him. Allison likes him at first, thinks he is funny, but he quickly puts her off with choices that he makes and the way he reacts to her. His friends think he is funny to the point that they don't take him seriously. His teachers think he is a 'good kid'. Miss Narwin thinks he has potential but does not get him at all. His parents don't listen to him and his dad thinks he's talented and wants him to run track.
We can combine these different view points together to come up with an opinion of our own about Philip which is exactly what Avi wants you to do. You take how others think of him and react to him, you have how he feels about everything and you put it together like a puzzle. After all no one person is capable of giving you any side other than their own.