Lydia mostly tried to be inconspicuous and not be called upon. She wwas listening to everyone and had her head tilted so she looked interested, but she doesn't like being called on. If she was, she introduced herself as Lydia Montgomery and said hi to Professor Carrow.
She then said she'd never taken Muggle Studies before, so her knowledge was limited. She did know that their buses couldn't travel nearly as fast/far as the Knight Bus, their Healers would sew them up when they got hurt with needles and thread and that when they dressed up as witches they painted their face green.