User blog comment:B214/High School DxD EX/@comment-24053170-20160114063835/@comment-24053170-20160114211428

Considering how this series has been going so far, I seriously doubt it will end with issei graduating highschool, since you seem to have forgotten that Kuoh Academy has a college on campus as well, since Loup goes there.

So the series likely won't end with Issei just graduating. If anything it will either end with his official marriage to his harem or the birth of his child, as Ishibumi pretty much said it himself that is what would likely happen at the end.