It's no secret that enforcing English-only rules in an English language school can be difficult. This shouldn't stop your efforts to have English spoken in your school because speaking English as much as possible is to their benefit as learners. Here are a few tips on how to keep your students speaking English.
If possible, have a mix of language groups in your classes. Avoid having classes in which all of the students speak the same first language. Having a mix of languages in a class will reduce student ability to revert to a first language when communication gets difficult. It also forces students to interact with each other in English.