It's not just all about teaching anymore. You have to be entertaining as well. If can demonstrate both in the classroom, you are on the right track. Learn to draw the anime charachters. Learn some basic magic tricks. Learn how to make kids genki. This if you study, can be learned in a day. If the kids tell their parents that the lesson was OMOSHROKUNAI... It means that you were boring and it's your fault that the lesson was boring. Kids in Japan are good kids. They are super curious and will want your attention. Please remember this.
Loud voices, punches, kicks and the brutal kanchos will come your way. How you deal with these is up to you. First is by protecting yourself at all times by being aware of your surroundings. Luckily most classes are an hour or less and that is a great advantage. 30 - 40 minute classes are my favorite.
The school that hires you wants to make money as do you. If there is ever a problem that you can't handle, talk to the assistant or the manager. Then again if you can't handle a few dozen kids then perhaps you should find a new career or put on a suit and teach adults.
Enjoy the lessons because no matter who you teach, guaranteed everyone including 2 year olds will sense the mood you are in. Will some students make you angry? YES! Can you be professional about it? That's up to you. The classes that I teach have around 30 students in each of them. I have taught classes with more than double that. Add the parents on SANKANBI ( OPEN HOUSE DAY ) and now we have more than 100. Funny how the kids seem to behave better than usual that day. Anyways my friend here are some teaching tips.
There is so much you can do my friends. Do it right and you will earn lots of yen and make kids happy at the same time. Best of luck to you.