One could argue that the book's abiding theme is the power of lies. We like to think that the truth is more powerful than its opposite, and yet, as the story ably demonstrates, the reverse is quite often the case. As the old saying goes, a lie can travel halfway round the world while the truth is still putting on its shoes. And even when the truth finally does come out, so much damage has already been done by lies that it's almost too late for it to make much of an impact.
That certainly seems to be the case in Nothing But the Truth. Philip's lie—that he was prevented by a vindictive teacher from singing the national anthem—has been seized upon by a sensation-hungry media and blown out of all proportion, with the result that the reputation of a perfectly good teacher, Miss Narwin, is ruined by all the adverse publicity.
Some basic research would've shown that Philip's story was completely false. But because so many people, especially those in the media, wanted to believe in it, it was never seriously challenged. This demonstrates that when the maintenance of a lie is in the interests of powerful groups and individuals, then the power of falsehood can become almost unstoppable, taking on a terrible momentum all of its own and destroying everything in its path, including the truth.