Wow 
Cool chart, finally an explanation for Freshman-Senior 
That was my exact thought reading that chart too xD
It always confused me.
Australia's states have been slightly different until recently (for which grades are considered highschool for starters)
When I went to school it got broken down into:
prep - preperation year, ages 4-5.
Years 1-7 : primary school . ages 5-11/12
years 8-12 : highschool. ages 12-17/18
That used to be correct. However I believe my state now considers year 7 as the first year of highschool, which now lasts 6 years instead of 5.
Once a student reaches year 10, they are given the choice to either continue schooling to graduate highschool or to find a trade/apprenticeship and work instead. Yes, our apprenticeships/interns have a set minimum wage for each year of experience (it increases from 1st year to 4th year).