Well, from what I partially know.
Students in Japanese Schools aren't allowed to Date with eachother by School Rules. And the few Schools that do allow, they need to get permission from Teachers & Parents to do so. So, majority of the times, couples actually keep their relationship a secret for a long time, because if they reveal it, not only would it be considered as social stigma but also people around might force them to break up.
Plus, relationship during School days is usually frowned upon in Japanese or any Asian society for that matter (which is why there are so many School Romance genre Novel/Manga/Anime series out there in Japan, as it's basically a fiction), as it is considered to be a distraction to education and sports as they lead to career building while relationship is seen as waste of time and immoral.
This is why there is actually a big interaction rift between boys & girls in Japanese Schools which leads to them acting seperately without ever interacting with eachother even in co-ed environment. And this is also a leading cause in lower marriage in Japan, as people are actively being discouraged from having relationship by the society form young age (as the common notion has become to avoid dating as it's a hassle).
Since the situation is commonly understood by people in Japan, Authors don't really explain it in their works.