A fourth grade class places a boys-against-girls bet to see which group can be the first to make their substitute teacher cry. The story is told in the point of view of Hobie, one of the kids in the class. As Hobie, his best friend Nick, and two girls in the class, Molly and Lisa, are giving a creative report on China, Mr. Star, their regular teacher falls ill with the flu. The principal stays with them for the rest of the day, but the class is overjoyed to learn that the next day they will have a substitute teacher. Mr. Star hasn't been absent all year, and with a sub the class can act up and do what they want all day.
The next morning at their lockers, Hobie and his classmates come up with a plan. Molly, the quintessential mean girl of the group, comes up with the idea of it being a boys against girls competition. The kids have trouble coming up with a prize for the winners until Nick says that the losing group should have to fetch the "pit balls" from a disgusting stairwell (the "spit pit") that everyone spits and throws trash in. Pit balls are balls during a game of kick ball that land in the spit pit.
The sub turns out to be Miss Svetlana Ivanovitch, a young, naïve woman who makes the mistake of confessing to the children that this is her first substitute teaching job. The girls start off right away with the old switching names routine. This causes the whole class to laugh, but Miss Ivanovitch isn't close to tears yet.
Hobie and some other boys lie to get out of class so they can plan their next strategy. When they come back, they find the entire class searching for Jenny's phantom contact lens. Not to be outdone, the boys tell Miss Ivanovitch that Marshall, an African American student wearing a San Francisco souvenir shirt, is Japanese. The story is that he only knows a few words of English and spends all his time folding origami (which Marshall is genuinely talented at) and dreaming of Japan. Molly keeps score of the game on the blackboard and while she is at it, changes the sub's name to something less flattering. Hobie worries to himself that they are all going to be in a lot of trouble when this is over.
Recess time. Both groups use this time to discuss their next moves, while Miss Ivanovitch decides to try to start fresh. The girls gather under the slide to talk while the boys struggle to come up with ideas across the playground. When it's time to go inside, the girls (all except one) throw a snowball at Miss Ivanovitch when her back is turned. The teacher retaliates, however, by gathering up some snow and pitching it perfectly against the slide. Once inside, Miss Ivanovitch changes her tone. She had been speaking to the kids as though they were kindergarteners, but now she has realized that fourth graders are a bit more clever and conniving than she gave them credit for. She acknowledges the morning's shenanigans and asks that a truce be declared. The kids all stare at her blankly, except for Aretha, who smiles back. Nick announces that it's time for music class. Miss Ivanovitch wisely checks the schedule first. While the class is at music, she watches wistfully how they behave under the supervision of the firm but fun music teacher.
The kids keep up their game. Miss Ivanovitch even gets into it and adds "Sub-two points" to Molly's scoreboard. During gym one of the boys accidentally injures Miss Ivanovitch while square dancing, but even this doesn't break her. RX left the classroom sink running all during gym class so that when the class returns, the room is flooded. It is during the cleaning up of this flood that Miss Ivanovitch laughs so hard that she cries. It is vaguely stated that the boys won the game since it was one of their antics that pushed her over the edge, but everyone (except maybe Molly) is sick of the game by that point. Eyeing the flood damage, the principal reluctantly asks Miss Ivanovitch to return the next day as Mr. Star is still sick. It seems that Molly will also be home with the flu, so Miss Ivanovitch anticipates a better day.
There are several sequels to this book dealing with the same group of kids. This author really knows how kids think and talk. Sometimes it feels like you are sitting in on a classroom when reading these books.