Why was Holling able to buy the cream puffs so cheaply? Why did his plan go wrong?

The answer to this question can be found in the November chapter. Holling needs to buy the class some cream puffs to make up for the ones that he accidentally tainted with chalk dust, which caused all of the kids to get sick, and they threatened him if he didn't get them some clean cream puffs. The problem is that Holling doesn't have enough money to buy from the baker. Holling attempts to trade labor for the cream puffs, but the baker says that he doesn't need any work done. What the baker needs is somebody who knows Shakespeare.

"What I should really need," he said, "is a boy who knows Shakespeare."

Holling has just started going through works of Shakespeare on his Wednesdays with Mrs. Baker, so Holling announces that he knows Shakespeare. The baker is skeptical, but Holling proves his knowledge by reciting a few passages. The baker is impressed and gives Holling the cream puffs that he needs. Getting the cream puffs cheap is a win for Holling, but it does sort of backfire on Holling. Holling also agrees to be in a Shakespeare production that the baker is putting on, and Holling gets to play Ariel and wear tights with feathers on his butt.