Can you clarify exactly what it is that you plan to study at online school? I am not sure how the Korean high school system works, but the US is different to many other countries in that it is the transcript for all 4 years of high school that a college looks at for admissions, whereas many countries have a final school leaving exam that is the only or most important factor. I am not sure what you are planning to do for 1-2 years in a US online high school that will improve your chances?