Mrs. Baker was always ice to Holling; he just didn't recognize that for some time after his school year began. The Wednesday Wars by Gary Schmidt takes place during the nine months of Holling Hoodhood's seventh-grade year, and on the first day of school he is convinced that his teacher, Mrs. Baker, hates him.
Of all the kids in seventh grade at Camillo Junior High, there was one kid that Mrs. Baker hated with heat whiter that the sun. Me.
Of course this is an exaggeration fabricated in Holling's mind, but he believes it. Every time he looks at Mrs. Baker he thinks she is glaring at him; every time she speaks his name, Holling thinks he is in trouble. It is true that a few incidents do happen early in the year which cause a little friction between them; however, once they start spending their Wednesday afternoons reading Shakespeare, the relationship between Holling and Mrs. Baker begins to change.
Mrs. Baker does make a bit of a change, as well, eventually deciding that Holling is capable of doing more than cleaning erasers on the Wednesday afternoons they have together.
"Mr. Hoodhood," said Mrs. Baker, "we have been wasting our opportunities.... On Wednesday afternoons from now on, we will be reading Shakespeare together."
This really is the moment where their relationship changes. By the end of the novel, of course, Mrs. Baker has become more like a mother than a teacher to Holling, and he cares about her, as well. Holling still has random doubts throughout the year, but things make a significant change for the better when they begin to study Shakespeare together.