The Garrett Twins were members of the Gang; a group of students at Experiment House school, who used to delight in tormenting and bullying other students, including Eustace Scrubb and Jill Pole.
They, and the other members of their gang, were never punished by the Head-teacher of the school for their antics on other pupils because the Head believed that children should be allowed to do what they wanted, and thought their gang were 'interesting' psychological cases.
They were presumably among those bullies who, in an ironic turn of events, were briefly tormented by Eustace, Jill and Caspian X, after Aslan sent them back home after their journey in Narnia. The Garret twins were presumably later expelled from Experiment House.