(Most of the time Kindergarten is part of elementary school but it can be in a separate school):

grade 1 to grades 6 or 7 (varies from state to state) ….ages 6 – 12

Secondary High Schools:

grades 7 or 8 to grade 12 …ages 12 – 18?

Senior Colleges:

grades 11 and 12 only … ages 17 and 18

I don’t really understand why there’s a separate college for 17 and 18 year olds while they can still be in secondary school, maybe i misunderstood it.

I think that Australlian schools do work to some degree like American schools do…

Primary/Elementary School
Kindergarten and Grades 1-4 or 5 (ages 5-10 or 11)

Middle/Intermediate School
Grades 5-8 or 6-8 (ages 10 or 11-14)

High School
Grades 9-12 (ages 14-18)

Or in some places, they have a system that’s a bit different:

Elementary/Primary School
Kindergarten through Grade 6 (ages 5-12)

Junior High School
Grades 7 and 8, or occasionally 7-9 (ages 12-14 or 15)

High School (might also be called Senior High School to distinguish from jr. high)
Grades 9-12 or occasionally 10-12 (ages 14 or 15-18)