What is ironic about the way Philip's journal and Ms. Narwin's letter end up on page 70 in Nothing But the Truth?
Book pages differ from edition to edition of any given book, and although there are no journals or letters on page 70 of my particular copy to Nothing but the Truth, I am assuming you are referring to the letter and diary entry that appear back to back at the end of Chapter 11. The letter is written by Margaret Narwin to her sister, and the diary entry belongs to Philip Malloy.
Both Ms. Narwin's letter and Philip's diary entry express the exact same theme - rejoicing in the support that each writer thinks she or he enjoys in the standoff between each other. Ms. Narwin tells her sister how "gratifying" it is to know that her principal is going to give her "exactly the kind of support teachers need." She considers herself "lucky" to have such a positive administration standing behind her, and knows how rare such a situation is. Philip's diary expresses the same sentiment. He is thankful that his parents are on his side in his battle with Ms. Narwin. He, like Ms. Narwin and her administration, considers himself "lucky" that his parents are "different" from those of other students. The irony in these two declarations is that both Ms. Narwin and Philip have identical feelings about the unusual support they think they have, even though they are on completely opposite sides.
The irony of these two positions deepens as the story develops. Neither Ms. Narwin nor Philip achieves a happy resolution, despite the support they each are so happy at this time to have. Ms. Narwin ends up being wrong about the level of support she will receive from her administration, and although Philip's parents do not betray him, their unqualified support for his position enables him to achieve an objective which is not noble, and which ends up causing him and everyone else great pain instead of happiness.