Thirty schools were evacuated on Tuesday and Wednesday, including students that were doing their HSC at the time. For people that are not familiar with the term, HSC is basically the university entrance exam in NSW, so it is a big deal.
Currently, it is confirmed that the emails came from Eastern Europe, so like many things internet, it will most likely be a black hole and we never found out who did it.
Now some personal experience, this happens frequently on both secondary school and university level. Most of the time around the end of the term examination period. I think this alone gives some hint on who benefits most from these type of incidents and the likely culprits.
I have never heard any of these turned into serious incidents. However, in the post 911 era, these tend to be taken very seriously all around the world.