I for one went to a school with uniforms, and I can safely assure you they would never let any permanent student, whether they were local or international, get away with not wearing the uniform. I can't speak for any other country (New Zealander here), but my experience is that schools are usually loathe to let anyone break their rules, no matter how silly they seem to anyone else. And based on 7 years of learning the Japanese language and culture, I would say they're one of the last cultures that would act liberal about this sort of thing.

But at the end of the day, it's the GMs decision as to whether the school is the kind to make a big deal about it. If it's a private institution, then it likely has free reign to determine it's own rules. If it's public, then I'm not entirely sure, though I would guess the government would be entitled to tell them whether or not to make a uniform compulsory. Whichever it is, it's the GM who controls them, and thus makes the choice.