Hi Sarah
Why did your Mother and you admit to speaking Japanese at home on the declaration? It's not like the principal can call your Mother a liar if both of you deny the fact that you spoke Japanese at home with your Mother and said you learnt all your Japanese skills from the internet, manga and anime, the system can not get you. This is the biggest loophole we Asians can exploit.
PS, don't be naive, Japanese population is one of the smallest ethnic group in Australia. Italians, Arabs and Greeks and many times bigger than the Japanese population and they have higher birth rate too, there must be other reasons and it's obvious.