Board Thread:General Discussion/@comment-94.0.112.90-20170226202217/@comment-26397825-20170322035115

At most, a reasonable level of retaliation in self defense to a bit of ear pulling would be slapping his face or hitting his hands. She could have done those, but there was no guarantee it would actualy stop the quartet.

It's not perfect, but it's a reason why she didn't retaliate.

Of course, the out-of-universe reason is because she wasn't a second year at that time. She was a random background character who only got fleshed out because the fans loved her so much.