Cassidy is a student in Grove Hills, and one of a group of three (with Graham and Lara) that Daria and Jodie meet in "Gifted". The three students are presumably friends.
She initially seems to be someone with a similar attitude to Daria, but as the evening goes on she shows herself to be obnoxious and dismissive of anyone with lower intelligence. She takes this to the extreme of shunning David as an "idiot" for "only" getting the 90th percentile on an SAT test.
She talks less than Lara and Graham, and disappeared after the encounter with David. Why did she leave? Did Daria's remark about tests showing "who your real friends are" hit close to home, is she not as close as Lara and Graham are, did she have to do something else, did the writer or animators want to have one less character to juggle in the later scene??