Edith Winterblott was a student at Experiment House school, in the autumn of 1942. She was also one of bullies in The Gang who used to bully other students, including Eustace Scrubb and Jill Pole.
She most likely spent most of her time at school bullying other students, and talking to the Head-teacher of the school (but not getting punished, due to the Head believing that she and her gang were interesting psychological cases), until the time when Aslan, Eustace, Jill and Caspian X came to the school, and frightened The Head and all the other bullies off.
After The Head was sacked from her job, Edith was most likely expelled.