What is the lesson learned during Scout's first day at school?
Scout learns several lessons on her first day of school in To Kill a Mockingbird. First, she learns that she is way ahead of the other kids academically: She can already read and write. Secondly, she learns that her new teacher, Miss Caroline, has some fancy new educational ideas that, when applied in a classroom, do not always work properly. Most importantly, Scout learns from her talk with Atticus about how to "climb into his skin and walk around in it." It is Atticus' way of explaining how to put yourself in another's position and try to visualize the situation from that viewpoint. Scout learned that Miss Caroline could not be expected to learn all of Maycomb's ways in just one day. Scout also learned that Atticus would support her, even if it came to going against Miss Caroline's suggestions.
It depends on what kind of lesson you are talking about. The teacher only wants to teach the kids about letters (a little bit) and to read them stories. So if that's the kind of lesson you're talking about, it's a story about a cat and a bit of learning letters in the morning.
But the lesson that Scout actually learns is that the teacher only wants things her own way and that Scout needs to not talk back to her.
The teacher ought to have learned a lesson about the way Maycomb's society works -- what the Cunninghams and the Ewells are like, for example.