high school diploma

in japan high school diploma issued by school exam or government exam

To earn a "proper" high school diploma in Japan, you have to attend a high school for 3 years. All you need to do during these 3 years is to pass school exams that you take about twice in each term (in most high schools). The context of the exams vary from school to school.

However, even if you can't do these things, you can get a qualification that equals to a high school diploma just by passing a government exam.
(This qualification is called "こうこうそつぎょうていどにんてい(高校卒業程度認定)" in Japanese.)
This qualification works pretty much the same as a proper high school diploma in most cases, though I guess proper one gives a better impression.

Sorry, the previous reply wasn't exactly correct.
During the three years, of course, you need to attend classes for at least two thirds of the school days in each year in addition to passing exams.
It sounded as if you don't even need to go to school at all except for exams... Sorry for the confusion.