For safety reasons it does make sense to have at least a few bilingual teachers on staff.
But above it does make sense to take the immersion approach to English instruction. And that means using it in all official settings - when conversing with teachers or principals, in the classroom, and so forth.
In the end, it's what's best for the children. If their English is poor, their prospects for success will be so as well.
It is real easy. You tell them to follow the rules. End of story.