Situational irony is when the opposite of what is expected happens. The first example of situational irony is when Scout gets in trouble in school for already knowing how to read.
Miss Caroline told me to tell my father not to teach me any more, it would interfere with my reading. (ch 2)
This is ironic because usually a teacher would be thrilled that one of her students was so far ahead, or that a parent was so involved. Miss Caroline is not!
Another ironic situation is the conflict between Scout and Miss Caroline over Walter. When Scout tells Miss Caroline why Walter does not have lunch, she is trying to be helpful. The class expects Miss Caroline to be grateful.
Miss Caroline and I had conferred twice already, and they were looking at me in the innocent assurance that familiarity breeds understanding. (ch 2)
Miss Caroline is not impressed. She thinks Scout is being snotty and asks her to hold out her hand (which, ironically, Scout thinks means they have reached an agreement) and hits her with a ruler. This whole situation is ironic because Miss Caroline punishes Scout when she is just trying to help.
Another example of situational irony in this chapter is the disparity between Miss Caroline and her pupils as a whole. The way that she looks, acts and dresses is at odds with the kind of class she is teaching. She is a very dainty, pretty sort of person (Jem appears to have a brief crush on her); Scout sums her up by saying that 'she looked and smelled like a peppermint drop'. However, her pupils are from a different, rougher kind of background, and she is not really able to connect with them as a result. This is neatly illustrated in the incident when she reads them a pretty little story about cats that talk and dress up. She entirely loses them at this point. As Scout reflects:
Miss Caroline seemed unaware that the ragged, denim-shirted and floursack-skirted first grade, most of whom had chopped cotton and fed hogs from the time they were able to walk, were immune to imaginative literature.
These children, then, are plainly-clothed, used to hard graft and do not respond to the whimsical stories that Miss Caroline regales them with. Miss Caroline certainly means well, but she isn't able to appreciate the reality that most of her pupils live in, which appears as a rather different world to her own.