Well, this is always an issue, teaching strategy and I always fall into the arguement that 'it's not what but how that matters.'

In high school I went from a school where French class was taught via English to a high school where french was taught via French. I was shocked that my fellow students could speak! The students were asking questions in french and interacting with the teacher in French. I was intimidated and worried on the first day. The teacher came over and told me not to worry and that I'd get used to it. And I did and it was great.

Maybe you underestimate the flexibility of children and their ability to be and do in the environment they're in. Or maybe that style just doesn't fit you as a teacher. I don't think anyone is forcing you to do this and you can stand your ground on effectivenesss for you as a teacher using the other method. we all tend to teach how we learn. We see it as the best way and it's hard for us to teach another way.

Have you tried teaching a class all in English?

You'll also need to give up grammar explanation as a fundamental teaching tool and have to rely on teaching understanding and teaching meaning. You can still use grammar but just 'this to that' as a teaching method gets thrown out the window.

Also, where are you getting your effectiveness stats? I'd really like to read up on it?