to be honest, does there need to be an official statement for dating to happen? It feels very un-mangalike yeah, but I don't think it's that uncommon for pairs to form in real life just through the process of mutual understanding and spending time with each other, without need for special "announcement for the change in status" - I personally know several like that at least
as we all are saying here, in practice they already act pretty much as if they're dating anyways
that being said, this is a manga so I'm sure it's going to be explicitly addressed sooner or later
Depends on the country and culture as far as I understand. I live in Brazil and here most people will only be boyfriend-girlfriend if it happens an official ask. Before that people can go out, kiss, have sex, but they will probably not consider themselves boyfriend-girlfriend and chances are it will not be monogamous either
In Japan there is this "kokuhaku" culture where people need to make this official love declaration, but the funny thing is that for what I can understand from anime, people can accept these confessions even when they dont reciprocate the feelings, as a way of giving it a try. This would not happen in Brazil normally (but people could go out and kiss anyway to test, so in the end the difference is in how to call themselves I'd say)