Panel 1: As he, Dawn and Liz walk down the street to the bus stop, Christopher reminds Elizabeth they get on a new school bus this year and have a new driver and everything!
Panel 2: When Dawn wonders if the bus driver will be a lady or a man, Chris figures that it's gonna be a lady driver so they gots nothin'to worry about.
Panel 3: He goes on to say that you can do anything you want on a bus a lady drives 'cause all THEY do is yell at you. As the bus pulls up, Dawn is about to name an exception to said gender profiling.
Panel 4: When the door opens and we see a generic old lady frowning at them, Liz looks as if she's about to void where prohibited as she yells "MRS GRUNION!!!" because she's the exception Dawn wanted to point out.
Summary: Given that the woman's cruel tyranny consisted of underreacting to an active threat to the safety of herself and her charges, I should think that Liz is primarily reacting to the woman's stern appearance and tone of voice. Judging from her nonsense about ice machines and jealousy power, it's not something she outgrew.