Keep in mind that situational irony means what happens is not what is expected to happen. The most obvious example in chapters 1-3 are on Scout's first day of school. It is expected that the adult (and the teacher) would have things in the classroom under control. It is also expected that the children in the room would look to her with a general sense of respect for her position. Instead, the children pity her. They know instantly that she is out of place, therefore, they cannot respect her authority. But because they are children, most of them have no idea how to remedy the situation. It is situational irony that Scout, likely the most precocious child in the room, must correct and "educate" her teacher on the social ways of Maycomb.
Certainly, Miss Caroline herself believes this "lesson" from young Scout to be inappropriate, and punishes her with several "quick little pats" on the hand with a ruler and a time out in the corner. It is again situationally ironic that just before this Scout expects to be sealing a deal with spit and it takes her a moment to realize she is even in trouble for what she has done. The irony continues when she speaks to Atticus about her confusion of the situation. Scout, though young, is an old soul. She does what she believes is right and acts out of a sense of social intelligence that she is far to young to be given credit for.