I am british and speak Japanese, I have recently completed a CELTA, so I a very new teacher (so this isn't really from my experience). But when I asked my trainer what they thought about me speaking in Japanese to the students. They said that it should only be used at elementary level so give instructions for exercises and nothing else. I know this is a different situation but I agree with you, they need to hear as much English as possible. My French teacher use to speak French and English to my at School and I can't remember a word, stick with it and they will get use to it.