How many people go to high school in Australia?

There's no single, readily available figure for the exact number of high school students in Australia. The number fluctuates yearly and depends on how "high school" is defined (years covered, whether including TAFE/VET participation).

To find an estimate, you'd need to consult data from the Australian Bureau of Statistics (ABS) and possibly state-level education departments, looking at student enrolment figures in years 7-12 (or equivalent). Even then, the numbers will be an approximation.