Responses
Nowhere, he is in high school in the beginning in the last one he graduate and they have sex for the first time because "he is an adult now" and in the extra he is in uni and they plan to live together when he graduate uni and get "married" in Japan (the adoption thing that gay couple's do because gay marriage is not legal yet in Japan)
Hope this help the confusion
the real question here is why is the uke 20 in highschool, couldn't the author just make Ryou a university student.....like why they making the story sus for no reason