Make your expectations clear from the start, but be patient – it can take several months of consistent effort before students really get it. I prefer to gently lead students in this direction by making them feel good about the challenge . i.e. I’m always sure to praise students in front of others when they make a good effort to use English. I point out (non English) chatter, without going overboard to the point where it affects flow. In other words – don’t spend a whole lot of time and energy trying to achieve this goal overnight, but gently push kids in the right direction.
Students can also be trained to catch each other out. If you use the phrases “English only please!”, or “Don’t speak Japanese, please!”, your students will start using them too with a little encouragement. It can also help to periodically get an assistant to address your students and their parents in their native language about what you are doing and why it is important.