Yeah, 10th grader and sophomore are interchangeable terms.

That's interesting, never heard of such a system before.  In America, we have preschool (optional), then elementary school is kindergarten, and first through 5th grade (6 years); middle is 6th through 8th grade (three years), and high is 9th through 12th (four years).  How many years of elementary and middle school do you have in Australia?  I also know that other countries sometimes use the terms grammar school and primary school, which I don't really know what that means.