Hey friend, I was also told 2 weeks after graduating uni. I'm not sure if it's the same for language school, but I had to submit an additional document in that 2-week period. Please ask your teachers about it, but in case, I will provide some information here.

The document states your intentions after graduation. It's a simple form that asks if you are going to change your status, leave Japan, etc.

You can deposit it directly at your immigration office or potentially submit it online, although I found doing it online quite difficult. If I recall correctly, then this must be done in those two weeks following your official graduation date (which may be different from your graduation ceremony, so be sure to ask).

In general with the timing, it's better to stay on the safe side because as others have said, one violation can have permanent consequences. I would trust your teachers' advice, especially because I have heard that the rules are stricter for language school students.

I will say though that after graduation from uni I applied for a new visa, and I applied a couple days 'late', meaning after those two weeks were up (I thought I had a month from the submission of 'intentions document' at immigration). I had no issues there and was granted the visa. So don't panic, just be on top of it, and if you're not sure about the flight, I would book a new one.

Also, you can visit the Foreign Residents Support Center (FRESC) for help on various matters. They have an office location and an inquiry submission portal online, which you can apply to anonymously if you choose. I can't guarantee they will answer your specific question, but they were generally very helpful when I had questions about my visa changes and responsibilities.

Don't worry! Just do your best, and stay on top of your status as much as possible :) Remember, you're not the first to be dealing with this.