Ms. McDonald is a former teacher of Sheldon Cooper's. It is unknown which grade or which subject she taught Sheldon.
She is mentioned in "The Junior Professor Solution" in which Bernadette was pressuring Penny to study for her new job, saying it "couldn't hurt" to which Leonard asked "Who here has ever been hurt because they were the teacher's pet?" before Amy, Sheldon, Howard, Leonard and Raj all raised their hand.
Sheldon then went on to say "It's as if the rest of the class wanted Ms. McDonald to forget the quiz."