I want to study in Japan, so do I have to go after graduating from junior high school?

Like you, I don't have any relatives in Japan. But I have friends in Japan. I go to high school in Japan. You just need to find a school first. Because Japanese high schools can't go to school if they want to. So this is not easy to find. I found it for me because of that friend. Japan cannot handle immigration. So don't think about immigration. High school in China mainly needs your report card and diploma. Just like entering colleges and universities, the important procedures in Japan are similar to those in China. You have to get a passport first, and then find a high school (you can go). If you find a school, you should be able to go to high school. But as far as I know, there is no intermediary to introduce high school.