There are guidelines, varying from state to state, that dictate what is required of schools seeking to expel a student ...
Informal expulsions run rampant in Australian schools, evidence suggests
Expulsion is meant to be a last resort, reserved for cases where student behaviour is so exceptionally difficult that it is impossible for them to stay at school.
A 2017 Victorian Ombudsman report found that there was reason to believe informal expulsions were more common than formal expulsions in the state.