They didn't realize they're already dating. It all progressly so smoothly, they somehow think everything is still the same as day one. BUT he's currently always expecting her to arrive be after school or on holidays; he's the one who ALWAYS let her in; they exchanged Line to be able to be sure they're alredy in the room; he lets her do as much as she please while together; she purposely is careless as she wants to him to ogle her; and now they tease each other.
At this point, the only way to break the status quo so they finally do something to make it official would be for a friend of them to watch or join them one day only to point out how close they act... or one of them to suddently begin to show interest for somebody else or get a girlfriend/boyfriend, as there's no way the new guy/girl would allow or be ok for them to still act like that. but that's something that will probably never happen and if it does, it would be till the very end