What happened in the woods, two years before?

Lissa and Rose were drinking out in the woods, talking about boys, when Ms. Karp caught them. On the way back to the academy, they come across a dying raven. Lissa pets it, and brings it back to life. Ms. Karp is upset at this, and tells Lissa that she must never use that power again, and that they must never speak of it again.