Referred to Nelson Boys, or Boys College, (because there is a Nelson College for Girls), Nelson College takes boys from age 15. Though called a college, it is a high school, not tertiary/university. We've not quite come to grips with the different names applied to NZ schools, but here's the Ministry of Education blurb. There used to be Standard (number) and Form (number), which have been replaced by Year (number) a few years ago. Children start going to school sometime during the calendar year they turn five, and compulsory education (Year 1) starts when they are six.
A school can be called (name, often location of) School, Primary, Intermediate, High School, Prep School, and Grammar, the last I understand also to be also high school.
Of special note, there are Kohanga Reo and Kura Kaupapa Maori, Maori immersion kindy and schools, that run parallel to the main stream system; these have contributed greatly to the resurgence and survival of the Maori language. Related Wiki here.