Junior high is a pivotal time for growing up and learning lessons, and Holling Hoodhood learns many lessons in The Wednesday Wars by Gary Schmidt.
One thing Holling learns is that he has a right to make his own choices about who he wants to be in life. He gets made fun of for playing Ariel (a fairy) in a Shakespeare production, but he survived the tormenting and could be in another play if that's what he wants to do. His father is certain that Holling will become an architect, but Holling is finally able to say that he is not sure who he is, yet, but he will let his father know.
Another thing Holling learns is that he loves his sister very much. While he and Heather spend most of the novel bickering with one another, he is the one who finds a way to go get her from the bus station when she wants to come back home after running away. Oh, and he saves her life.
Holling learns that he can be successful at things if he practices. Mrs. Baker works with him on his role as Ariel, and he improves dramatically. She also teaches him the proper running form, and he is quite successful at running cross country.
Perhaps the most important lesson Holling learns is not to make such quick judgments about things. On the first day of class, Holling dramatically pronounces that
[o]f all the kids in the seventh grade at Camillo Junior High, there was one kid that Mrs. Baker hated with a heat whiter than the sun.
This Mrs. Baker is the same woman who gets him tickets to Opening Day at Yankee Stadium--and then takes him there herself when his father will not. Who comes to see his play when neither parent will attend. Who takes him to the hospital when his parents aren't concerned enough to leave their television program. He judged Mrs. Baker too quickly, and he misjudged his interest in Shakespeare much too quickly, as well. Soon he is really enjoying (mostly) his reading and even uses Shakespeare in his real life. In both of these things, Holling made a hasty judgment which proved to be false.
Several other characters learn lessons, as well. Meryl Lee Kowalski's father learns that it does not pay to cheat and is rewarded for his willingness to admit he was wrong. Mrs. Bigio learns that grief makes people do and say some very unkind things; however, after some time to reflect she makes amends to Mai Thi in a grand and wonderful way. Heather Hoodhood learned that she was not quite as ready to be independent as she thought and her brother is not such a bad kid after all.