Why was Callie even in that class?

My guess is that because she got pregnant, they threw her in that class until it was time for her to transfer. Either that or she was flying under the radar and wasn't putting forth effort to show her intelligence until LouAnne's class.

Just a guess, though.

Could also be cause she was working night shift at the supermarket every day, something that might have kept her back comparing to the more advanced classes.

A lot of kids are smart, but don't apply themselves. She was pregnant and working a part time job. Maybe she wasn't motivated to do any of her school work until Miss Johnson came along.

She might have been just doing enough to get by and not stick out. It seems she connected with Ms. Johnson right away and wanted to participate in class, a right teacher will ignite a work ethic in a satisfactory student.