* The teacher, Miss Caroline, was unfamiliar with Scout's background and learning: Miss Caroline was a young, inexperienced teacher from the North who didn't understand the culture and educational system of Maycomb. She dismissed Scout's knowledge of reading and writing, assuming Scout was "too advanced" for her class. This made Scout feel underestimated and frustrated.
* Miss Caroline's strict teaching methods clashed with Scout's natural learning style: Miss Caroline emphasized rote memorization and phonics, which didn't engage Scout. Scout was a self-taught reader, already capable of comprehending complex texts. The rigid approach stifled her curiosity and made learning feel boring.
* Miss Caroline's interaction with Walter Cunningham was a humiliating experience: Miss Caroline's patronizing and condescending attitude towards Walter Cunningham, a poor boy who couldn't afford lunch money, made Scout uncomfortable. It also revealed the social divisions and inequalities present in Maycomb, which Scout was already aware of but hadn't explicitly confronted before.
In essence, Scout's first day of school was disappointing because it was a clash of worlds. Miss Caroline's approach was alien to Scout's experiences and understanding of learning, leading to feelings of discouragement and frustration.