score:4
No problem at all in South Korea. You can come an go at your leisure and unless you are blatantly violating a hotel policy or local law, no one will care. Rules may be slightly different for minors but that's more up to the chaperones or parents, then the hotels
In fact, for most "normal" bookings, you could stay in the room of your girlfriend without having to buy an extra room. Some hotels may charge you a little extra depending on the time of reservation but most bookings include 2 people, even if you book as a single person.