Some will insist on it- but not all. I’ve worked at 3 schools and none had an “English only” policy.
I went on demo once and a kid couldn’t remember the English word he needed. I told him to tell me in Chinese. The manager/director interrupted and said only English was allowed. Another kid could supply the word for him, or he could act it out. But no Chinese was allowed in English class.
I once had a manager tell me the children were speaking Chinese too much (at play time). I pointed out to her that she was speaking Chinese to them all the time, and that it certainly didn’t promote their using English. I didn’t hear about it again. I left shortly after for another reason.
How old are these kids? Do they have enough English to communicate? If they don’t then it can hardly be expected they either use English or stop talking. More info pls.