Charging anything makes you run like a business, and that's not necessarily going to change the way anyone teaches. And many of us actually expect monthly charges. I don't want to have to pay every time I walk into a school if I have a choice to pay for the whole month in advance.I would recommend don't charge anything per month and don't teach martial arts as a money making enterprise.
If you do need to charge to cover your time and/or premises then charge by attendance. Monthly charges are a big turn off to many people, and they make you a business, which changes the way you teach.
Those differences may all be due to cultural differences. In the US, charging by the month is the norm.