I need to write a two-sentence summary about The Wednesday Wars.

I don't think that a two-sentence summary is possible to write about this book, because far too much happens over the course of Holling's school year to be adequately summarized in only two sentences. Holling's school year includes his relationships with his friends, his sister, Meryl Lee, his father, and Mrs. Baker. Each of those relationships deserves more than two sentences. Additionally, a lot happens to Holling—things like his rescues of Heather, the escaped rats, and the poisoned cream puffs are all integral to the story. I think it is possible that this question might be asking for a "teaser/tagline" summary. This is different than a full summary, because it's not meant to summarize everything that happens in the story. A teaser summary is something that you would read on the back cover of the book. It's meant to get a reader excited for the book, and it does this by sounding exciting while remaining vague. If that is the case, then something like the following might work.
"The Wednesday Wars tells the story of Holling and his struggles through 7th grade year. Along the way, Holling's life is filled with and affected by a hilarious host of supporting characters that will have you laughing out loud throughout the book."