In Nothing But the Truth by Avi, Philip gets a letter saying that his homeroom has changed. What do his parents take it as proof as?

When Philip receives a letter telling him that his homeroom has changed in Nothing But the Truth, his parents take it as proof that Miss Narwin does not like him. Earlier, Philip told his parents that Miss Narwin won't let him sing the National Anthem in class even though he knows Miss Narwin is following the school rules. Philip has further given them the impression that his grades in English are poor because of the teacher instead of taking responsibility for his own lack of studying. He nearly failed a test because he did not read the assigned book, and he thinks he can get by through his sense of humor. Miss Narwin, however, has high expectations for her students, and even writes Philip a note telling him she knows he can do better. She gives him a second chance, but Philip would rather blame her than try harder. He basically lies to his parents, and they believe him. In other words, he has given his parents evidence (though false) that Miss Narwin has something against him, and the homeroom change just helps seal his story for them.