For example, if a teacher just got out of university, and say he turned out to be quite a good teacher at a particular high school, how long until they would let him teach ‘seniors’?

i.e. in Australia, the last 2 years of high school are Year 11 and 12

I noticed that some teachers gradually work their way up to teaching ‘seniors’. Can exceptional teachers jump straight to teaching a Year 11 or 12 class?