User blog comment:B214/Volume 22 spoilers/@comment-90.202.240.151-20161031050614/@comment-9595671-20161031102217

It's likely Great Red's title it references since he showed up first and fits more with the Highschool part. Especially since Issei second body and a small bit of his power was made from his flesh.

Not just that, there's also the fact that with all the dragons in the series, it's in a way saying that Issei is the Highschool Dragon of all Dragons. Aside from Saji, no one else with dragon powers is actually a Kuoh Academy student.