I noticed that on Japanese 履歴書s, graduating from graduate school is marked with 修了 instead of 卒業 like for undergraduate and primary/secondary education. I'm curious as to the reason this difference exists and in particular why the distinction was made in the first place.
There's a post (in Japanese) about this at: http://wonder-trend.com/archives/3651.html
As I understand it, the reason basically is because in the context of a CV you use 卒業 to indicate graduating (completing studies) from a specific school and you use 修了 to indicate completing a specific program/course of study. See also: this goo dictionary comparison of 卒業 and 修了
For instance, you may write:
to say that at Time A you entered into a master's degree program at a certain department in a certain college in a certain graduate school, and at Time B you completed the master's degree program.
If you want to use 卒業, you could say something like 大学院を卒業した but without context it's not clear if you mean you got a master's or a PhD then, or what the subject was, and of course you want to be precise and pro forma on a 履歴書.
It's really not that different from English: You graduate from a school, but you complete a degree or program. One often talks about graduating from college without too much concern about the precise degree (BS, BA, etc) or major. And in casual spoken conversation among fellow math PhDs I may say I graduated (from grad school) in 2004. However, on my CV or to someone who doesn't know what degree I have, I write "PhD, 2004" or say "I got my PhD in 2004."
The word 卒業 usually means completing a predetermined set of courses or classes. Therefore, you can use 卒業 for 小学校, 中学校, 高校, and 大学. However, since 大学院 does not usually have a predetermined set of courses but each student can pick and make his/her own plan of study, the word 修了 is used for such cases.
It's because most people go to graduate school expecting to earn a masters degree or PhD but few go the full course. You earn a degree by taking required classes, doing original research, writing a thesis and (successfully) defending it. If you go to grad school and do all these things but your doctoral thesis isn't up to snuff, then you can still say 修了 as in 博士課程を修了しました, but you'd be on thin ice if you claimed to have a PhD. If you earned a doctorate, i.e., you successfully defended your doctoral thesis at an accredited school and have the right to append PhD to your name, then you can put (東京)大学博士号取得 on your 名刺。There are some gray areas that you can read about on Japanese Wikipedia but the situation I have described corresponds to how the terms are used in Japanese society and media.
修了 means to conclude education, training in company.
Actually, in the end of every school year (middle of March), students in elementary school, junior high school and high school do 修了.
卒業 means to graduate from school and to leave an idol group.
But graduating from graduate school is called 修了 more often than 卒業.