There are many conflicts from which to choose and I think a case can be made for many of them as representing "the main conflict." Critical conflicts confront Eliza. One such challenge is how she is seen before winning the bee and afterwards. Teacher view her as a lazy and apathetic student before she wins, and after her triumph, they have a new found respect for her as she represents "a winner." Whereas she was seen as a loner before her victory, her glory affords her a seat with the other smart students who, like the victor Eliza, represent TAG (Talented and Gifted) students.
There might even be a conflict within herself, in terms of the way she perceived herself before the bee and afterwards. Another set of conflicts involve Eliza and her place in a fragmented family. Certainly, prior to her victory, there as a gulf between her and her father. The victories as a speller now brings them closer, which causes some a level of consternation within her as it comes at the expense of the acceptance of her brother.
Finally, when Eliza deliberately misspells a word in the next year's bee, it represents a conflict within herself as she knows she is giving a wrong answer. It also represents a conflict within herself because she is afraid of the person she is becoming, the person her father wants to move closer to God. These conflicts range as representing dilemmas between Eliza and her world, Eliza and individuals in her family, as well as Eliza and her own sense of self.