Language schools teaching in Japanese?

Hello all. I've been looking at going to a japanese language school for awhile now but I have a nagging question about the whole process. How do students learn Japanese from scratch when they teach IN Japanese? I know it's done but it just seems hard to rap my head around. I wouldn't be able to converse with my teacher or anything. So how does this work? How do you go from zero to learning in an enviroment you can't understand? Does anyone here have experience?
Thanks all that responds.

Teaching method using only the target language (direct method) IS used a lot around the world, don't worry :) When I went to a course in English language teaching in the UK, the instructor demonstrated how she would do that (using another language) - the students do a lot of guessing and logical thinking to figure out what the sentence structure or the meaning is. And they do fall in place in your brain properly.

I have also been trained to teach the Japanese language (1) through direct method (using Japanese only) and (2) using English language for explanation. Some students were fine with (1), others needed additional explanation in English, where (2) was useful. But I must say in classes (1) is prevalent - you need a lot of exposure to the language you are learning to absorb and acquire skills in it :) You will experience it, really, no worries :)

How do students learn Japanese from scratch when they teach IN Japanese?

Not sure if you realize, but not all schools teach IN Japanese. There are plenty of schools that teach in English or other languages should you prefer that type of learning environment.

Thanks for the response. I mean I know people do it and I've read plenty of success stories to know these work. It's just odd to think about.

@yllwsmrf
What Japanese language schools in Japan do you know of that teach in english?