This is a tough question to answer because the story is not told in anything close to resembling chronological order. It starts about as far into the timeline as it could go -- the competition. Then each member of the team has a flashback that explains a bit about their previous summer or beginning of the school year. They all fit together, because each flashback shows exactly how they came to meet and be a part of the team with their teacher.
Despite each flashback having a few minor conflicts, I would say that the overall main plot conflict is Mrs. Olinski's choosing of the team (The Souls). Specifically it is her struggle with exactly who should be the fourth member of the team. It may seem like a minor conflict, but I do feel that Mrs. Olinski choosing Julian as the fourth member represents the plot climax. After choosing him and forming the full team, everything else is more or less falling action and conclusion. If choosing Julian is the climax, then the search for Julian (4th team member) is the main conflict.
Here is that section of the text:
"The Souls continued their animated conversation, when suddenly, as if on signal, the four of them looked back at Mrs. Olinski.
And that is when she knew.
That is the exact moment she knew that Julian Singh would be the fourth member of her team and that she would always give good answers when asked why she had chosen them."