where to live while in school

would it be cheaper to live in a apartment or live in a school campus in japan for schools that teach japanese.

Normally Japanese language schools (as against universities) don't have large campus, unless they are part of larger colleges/universities, so they might not have on-campus dormitory facility available.

Prices would differ depending on exactly which location and what the school charges. But if you think about the convenience, if you rent an apartment, you will need to look around for an apartment on your own (maybe the school can help you finding one and also being your guarantor too) and buy some furniture, while if the school has dormitory available, you don't have to search for them, and they might also have some furniture already.