Alright, since some people like to label threads that have no planned participants and such with ((Open)) that can sometimes be confusing if others don't, yet the thread is actually meant to be available for whoever wants to join in.
My suggestion is this: If the thread is in a public location then assume it is open unless it's specifically marked as reserved for another character.
I'd also add that dining hall threads and the like (at least mine, don't want to speak for anybody else) generally continue to be open even if another character is already in there, posting. It's a very public place, feel free to join in!
I'd like to hear what everyone else thinks about this since I feel nobody should ever go "Hmm, I want to drop [character] into this scene, but there's already two people talking over lunch, better not disturb them." cause I feel that can inhibit RP and makes the school feel less open and alive. (The threads this doesn't go for tend to be fairly obviously private, imo)