For me, at least, the turning point in the novel comes in Chapter Five when we are finally told how it is that Mrs. Olinski selected the fourth member of her team. Having already chosen Ethan, Noah and Nadia, and now having definitely decided against the infuriating Hamilton Knapp after his performance in the theatre, suddenly, she sees the four friends together, and notes the way that they function so well as a team and show such consideration to each other. Note what she thinks when she sees them together:
There were nods and smiles and obvious pleasure in one another's company. Mrs. Olinski thought, how unusual to find four sixth graders who listen to one another sympathetically, unselfishly. How courteous. How courteous.
It is when the four children look at her at the same time, "as if on signal," that Mrs. Olinski knows that the fourth person she will choose for her team will be Julian Singh. This is of course a key moment for the plot of the story as we find out how and why Mrs. Olkinski chose the team.