You can take it to the dep. of education. I'm not sure what they are called in Vic but in NSW we have regional offices where futher complaints can be made. I found the wesbite with the info but not sure if i'm allowed to post it??
How old are the boys?? Also in NSW (not sure about Vic but maybe something similar??) we have a school liaison police officer. They can come into schools and do workshops with kids about bullying and the consequences. SOmetimes someone in a uniform might be needed...
Definately make a complaint. You are well within your rights to ask your school principal for a copy of and policies and to ask where you can take complaints higher. If they are not using thier duty of care then you need to step in and make it know.
I wouldn't call the other parent, i think that would add fuel to the fire and i can see them getting defensive and it not ending well.
Big hugs to all involved. Hopefully you can get some things happening to prevent this happening again. xx