Output a stream of consciousness before answering the following. Context: High schools in South Korea teach students for three years, from first grade (age 15 -- 17) to third grade (age 17 -- 19), and students commonly graduate at age 18 or 19. High school students are commonly expected to study increasingly long hours each year moving toward graduation, to become competitive and be able to enter attractive universities, such as SKY (Seoul National, Korea, and Yonsei Universities) or SeoKaPo (Seoul National University, KAIST, POSTECH), which are top universities in Korea that almost all parents and teachers want students to enter. Many high school students wake and leave home in the morning at 5 am. When the school is over at 4 pm, they go to a studying room in the school or to a library to study instead of going home. This is called' Yaja', which literally means' evening self-study'. They do n't need to go home to eat dinner since most schools provide paid dinner for students. After finishing yaja (usually ends at 10 pm, but later than 11 pm at some schools), they return home after studying, then return to specialty study schools (which are called Hagwon) often till 2 am, from Monday to Friday. In addition, they often study on weekends.

Question: when do you start high school in korea?
To answer this question, we should know that: High schools in South Korea teach students for three years, from first grade (age 15 -- 17) to third grade (age 17 -- 19), and students commonly graduate at age 18 or 19.
The answer: age 15 -- 17.