The teacher is the one in charge. Children learn from her.
I'd say that one of the most important lessons young children will learn is how to interact with others. That's a major part of social development. The daycare is letting the children down if they don't address that issue. There shouldn't be any excuse for such kind of aggression.

When I had a new kid enrolled, I made sure that the others welcomed her as "one of us." The children knew when there'll be a new kid in the daycare days before the new kid started.

Empathy. That's what children need to learn as early as possible.