Is it a cert 3 that you are thinking of? I'm not sure that there's a teacher aid specific course, but I believe the Cert 3 has teacher aid modules that you do instead of the childcare modules - so more targeted at the school system rather than the child care system.

Wanted to add that I don't think you really get to choose the age group you work with in the state school system - teacher aide jobs are pretty well snapped up as soon as they become available in my region - and usually by parents.