How to encourage students to use english in their daily conversation at school?

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There are many schools in my city which teach the school subjects in English. Therefore the students who join in the class should speak English. The problem is that they only use English to the teachers in class. During the break time or lunch time, they play and talk to their friends in their mother language. That's why their English is not fluent although they have been learning English for many years. The teachers already tried to encourage them by having English competitions and games, but there is no significant result. Do you have any idea to encourage the students use English?....