I would be making an urgent appointment to speak to the Principal yourself. You don't know exactly what the teacher told the Principal and they might have downplayed the incident. I think this incident is sufficiently serious that speaking directly to the Principal is in order if you are unhappy with the teacher's response. I would try at first to go in with a cooperative approach - you want to speak to the Principal about what happened and to work with the school in making sure that the impact on your daughter from this incident is minimised. I would also stress that you're concerned about the boy and whilst you don't want to know what's being done in relation to that (and understand its a private matter) you do want to know that its being treated seriously.
You can also speak directly with the Principal then about not talking to your daughter about this without you present.
I would also give the Bravehearts information and support line a call and seek advice from them on what to do in this situation, and what to expect in terms of effects on your daughter. There may not be anything immediate but down the track the incident may play on her mind. Bravehearts are incredibly supportive and are (unfortunately) experienced in dealing with school incidents such as this.