Yeah, this backstory may as well be it's own manga. Seriously, the small hints we have gotten so far are just to dramatic to let it go with " and then she was kidnapped, I was tortured. . you know, the usual meet the family night before the wedding"
@Ryomi@bejad007 not japan but my country in SEA has an exam that could be a replacement for your high school diploma. There's also a national entrance exam in which your score will determine which university of choice you can enter. Some universities also have their own entrance exam in addition if you didn't cut it in the previously mentioned exam. Although a requirement, a high school diploma is usually a formality if you take the exams, since only your score will be used to determine whether or not you will get accepted.
@bejad007 Seems like you can take/retake the exam regardless of highschool.
https://en.wikipedia.org/wiki/R%C5%8Dnin_(student)
https://en.wikipedia.org/wiki/National_Center_Test_for_University_Admissions
@Ryomi that exam is more of a special case for drop outs or those that doesn't have a formal education i guess. There's actually two types admission in my place, one is through invitation and another through the entrance exam I mentioned. For the first type, basically the university of your choice will evaluate your performance in highschool based on your report card and achievements i.e: extracurricular activities, competitions and organizational experience. If you don't get accepted through this process then you'll have to go through the other type of admission to get to uni. I think Japan more or less follows the same system or more likely ours is based on them.