When the family moves to Welch, Mary gets a job teaching (The Glass Castle 46). Mary doesn't enjoy teaching, and in order to keep her job, the children do a lot of the work that Mary should be doing as a teacher (creating lesson plans, grading assignments, keeping the classroom clean and organized, etc). It's during Jeannette's description of the help they provide their mother that Mary makes several comments about how smart her children are. Although the children are very intelligent, there isn't any evidence provided in the story that says they are considered "gifted" from an educational viewpoint. This background sets the scene for when Mary takes Brian and Jeanette to Welch Elementary and introduces them to the principal there. Mary tells the principal very proudly that her kids will be "two of the brightest, most creative children in America in his school" (85).
Two of the main reasons the principal doesn't believe Mary about her children's "giftedness" is because of the fact that Mary herself doesn't appear put together. She doesn't have any of the necessary documents or forms that the school needs, which shows how unorganized and incompetent she is. She doesn't provide any credibility for herself, so how would the principal believe her testimony to her children's intellectual aspirations? The second reason occurs when the principal attempts to test their math skills. In reality, he is asking the two children a very simple multiplication problem: "what's 8x7?," but to the children's ears, his accent is so thick that they can't understand him, and their inability to decode what he's saying is misconstrued by the principal as being "slow." Likewise, once Jeannette understands the question, and she excitedly answers, the principal cannot understand her accent. As the conversation continues on in this manner for a while, the principal comes to the conclusion that these two children are, in fact, "a bit slow and had speech impediments" (85). He places them in classes for students with learning disabilities, thereby providing support for not believing Mary's assertion that her children are gifted.