Why is it ironic that Philip, on Sunday evening, reflects that maybe he should go ask Miss Narwin for help.
Philip is no longer in Miss Narwin's class, so she can't help him.
Miss Narwin would never give him extra work. Not at any point in the book was she willing to help him.
It is ironic because it is the opposite of what he means.
He should have done this in the first place. The fact that he didn't has created all the other problems.