If you imagine that to make 250,000 a month profit from your business, you have to be pulling in at least double that so you can pay your overheads, salary, advertising, rent, and utilities Calculate how many students you need paying saying 3,000 yen an hour (say 12,000 yen a month, by 8 hours a day, 5 days a week to cover at least your overheads, never mind anything left over.You might have kids classes paying 20000 yen a month but that will be once or twice a week. You have to deal with student cancellations, finding new students, no shows.
I think noobie teachers flatter themselves that they think they are indispensible to their students and the students will simply change horses because you decide to quit and work for yourself. Some will but the majority will not, and you will also find students will quit on you on a dime. A lot of your time is spent networking and prospecting for new students. Do you know where to find them and how to keep them once you have got them? What will you use to draw them in or are you going to teach them out of Starbucks? Real professional.