Moving to Japan alone for 12th grade

Hello! I've recently visited Japan during my summer vacation. I was born in Okinawa and moved to America at the age of 2 so I have basic Japanese language skills. I can understand very well, and hold basic conversations, but reading and writing are definitely lacking.
I am currently 16 yo, and going to be in the 11th grade and I am considering taking my senior year in Japan as I have been thinking about this for a long time. Studying abroad is not an option as it is unaffordable, and housing is not an issue as I already have accommodations from a family friend.
I currently attend a Japanese language school in America and am learning Japanese, but if I were to move to Japan for High school I understand that I would need to know A LOT more than I do now. I am willing to make a serious effort this school year to greatly improve my Japanese, and if it is sufficient enough to successfully move to Japan and pass the Entrance exam is it possible?
Also, how would the VISA system work for a minor moving alone? Or I might be able to have my mom come with me as she already has a VISA.
Bullying and whatnot isn't an issue, I'm just looking for a way to make this WORK. Study abroad and expensive international school are definitely out of the question as they are WAY to unaffordable.
Thank you !

You'll have a very hard time finding a Japanese school willing to accept you for one year outside of any formal exchange program, if you are not already living in Japan.

You make me feel old, but here are some relevant questions for you to consider.

What is your medium term plan? doing one year high school in japan is likely to result in bad grades because you need to do your schooling in Japanese (even if you are smart, writing answers will be hard - you said so). if you are going on to university (in Japan), having bad grades might not be so good. Getting a mundane job it might not matter so much.

You mention money is a problem studying overseas, but you are going to be living alone or with a host family? That takes money too.

For reference, I have a friend whose son is "semi-lingual" because they spent five years in an English speaking country in their teen years and they missed a lot of Japanese schooling. He is now in a three year intensive/special high school program to bring his Japanese level back to where it should be.