Would this exchange student plan work?

I'm currently a junior in a high school in the USA, I am 16 years old (I have a summer birthday). I wish to do a homestay for my 12th grade year in Japan at a Japanese high school, but there's a twist.

After I finish my 11th grade year in the USA, I want to stay in the USA without progressing to 12th grade for about a year, basically take a gap year, but before I graduate high school. During that year I will work and gain $, and then I will return to high school a year later but instead I will be an exchange student for my full senior year and graduate in Japan. I will also talk to a counselor about this and what classes I can get credits for. I will be 17 from the beginning of the Japanese school year in April, 2022, until June, 2022, and I will still be 18 by time I graduate in March when I graduate. I believe I am doing this right. I turn 17 in June of this year, I will go there in April of 2022, turn 18 in June, then I will graduate in March of 2023 while being 18 years old.

Is anything stopping me legally from following this plan?

(Note: I'm not sure if this matters or not with an exchange student program, but I am certain that my conversational, reading, and listening skills will be good by time the exchange student program starts. I have been studying for about a little over a year, and am already at a good conversational level and reading level. And if I add an extra year to the mix, which I will be, then I should be set.)

I would be more concerned that you may not be able to take a gap year from high school - you would need to make sure that there are no legal issues with you leaving school at that age with a plan to return later.

You may be confident in your Japanese progression now, but remember that you would need to be fluent by the time you attend school. You may also need a guardian of sorts in Japan, as legal adult age there is 20 (not 18), plus a student visa (which I don't think is normally given to study at high school). Does your school operate an exchange program with a Japanese school? If not, I don't you'll be able to just enrol anywhere in Japan; attending high school would not be considered an appropriate reason to need a visa unless your entire family had moved there (i.e. you were your parents' dependent in Japan).

I'm in the US., I do not know of any high schools that allow a gap year. In general, most US schools are much stricter about absences now than they were when I was a child. Maybe if you go to some fancy expensive private school they have something like that on offer, but no public school. Now if your parents are on board with this they can offer to home school you and you can meet whatever the minimum requirements are for homeschool but you will then technically have graduated. It's also possible you could make a deal with your high school to complete your studies in Japan, but I just can't see a school letting you just take a year off to work. I think it's a lot easier to find a full year program in college.

There are also limits on how longer a 16-17 year old can legally work in the USA.

Teens ages 16-17 may work up to four hours on school days, eight hours Friday through Sunday and 20 hours during school weeks.

Neither age group may work more than six days in a row whether or not school is in session. When school is out, older teens may work up to 48 hours between the hours of 5 a.m. to midnight (during the school year they work until 10 p.m. or midnight on weekends). Younger teens may work up to 40 hours when school is out and up until 9 p.m. at night during summers.

Legally:
- Would you have a guardian in Japan?
- Visa to stay and study in Japan? I don’t think senior high schools sponsor students for their visa.

More practical consideration:
- Finding a high school that accepts you just for the senior year, and allow you to graduate from that school might be difficult.
- Your Japanese language proficiency level might not be sufficient. (Full reading comprehension and writing as well?)
- Senior year in senior high schools in Japan is a busy time for the local students as they prepare for taking entrance examination for colleges/universities, and might not be the best year to accept exchange student (for a full year, at least).
- There is no “credit” system in Japanese schools – each year you cover all subjects widely, so what you are planning in terms of “credit” transfer from your school to another one might not work.
- Please check with exchange program organizations – Rotary, AFS (are they still around?), etc., to see what they have available.

「Is anything stopping me legally from following this plan?」
What you're planning isn't illegal, but it's unlikely to work.

「During that year I will work and gain $, and then I will return to high school a year later but instead I will be an exchange student for my full senior year and graduate in Japan. 」
By "graduate in Japan" do you mean you hope to receive a diploma from the Japanese high school? That's not likely to happen.

The way exchange programs work is that the host country school allows the student to study at their facility, and the home country school agrees to recognize the academic work that the student did there. So if you're a student enrolled in a U.S. high school who goes to Japan for an exchange program, the agreement between the schools would be that you can study at the school in Japan, and the U.S. school will count the course work you did there towards your graduation from the U.S. school, and so your diploma would be from the U.S. school.

Are you perhaps talking about becoming a transfer student, enrolling as a full-fledged regular student at a Japanese high school part-way through your high school education? That's generally something that Japanese schools don't allow on a discretionary basis.

「 I'm not sure if this matters or not with an exchange student program, but I am certain that my conversational, reading, and listening skills will be good by time the exchange student program starts. I have been studying for about a little over a year, and am already at a good conversational level and reading level. And if I add an extra year to the mix, which I will be, then I should be set.」
You're not certain if the levels of your Japanese speaking, reading, and listening proficiency will matter for your goal of studying in a Japanese high school? Yes, they will. What language did you think Japanese high schools teach their lessons in?

I don't mean to belittle your determination and passion for the language, but if you think a total of a little more than two years of studying is all it takes to learn enough Japanese to function as a high school student, that's probably overoptimistic.

Honestly, I think you'll probably have a better experience finishing high school in the U.S. as normal, while continuing to study Japanese, and look into doing a program at a university or language school in Japan at a later date. Your desire to study in the country for a whole year shows a level of academic commitment that your language skills probably aren't ready for just yet, but it sounds like you might not be satisfied with the shorter, less intense programs you could more easily qualify for right now.