Can I study at a Japanese High school at 19?

Hello, I am a 18 year old High school dropout, and I will be turning 19 next month. I will be homestaying in Japan for about 3 months maybe, and I was wondering if I can enroll into HS. For some personal issues, I was unable to attend HS a lot, so I have a awful GPA, also, I had one last year of HS before I dropped out. I learned Japanese as a kid and I am probably at a n3-n2 level in Japanese. Also, money shouldn't be an issue as I am working myself to death currently. Anyways, can I enroll into a Japanese high school while homestaying or should I do something before leaving on my trip? Thank you for taking the time to read this. I hope you have a wonderful day!

Please ask your homestay host to see if they can find a senior high school near them that would be willing to accept a foreign student for a limited time. Senior high school is not mandatory education, and since you are 18 that places you in the senior year, when other students would be studying hard for a university entrance examination.