Cissie was in private school, I think after she competed in the Olympics. They were always weird about the YJ girls outside of Cassie appearing (until Cassie's mini). A lot of storytelling potential with them all at the same school. Could've kept Anita in for good measure.
Cassie really didn't have many close friends after Conner and Bart died, outside of Kara and Tim. Makes me wish we got a Wonder Girl ongoing instead of a mini.