Theory 1. (Sam meets Deanmon at the end of the 200th episode)
In the start of the episode Sam is still searching for Dean but has no luck, he get's a call from a hunter telling him about a disappearance of a teacher in a school of "girls" so he decides to take a break and to go take the case on. The episode would play normally as it did but at the end when Sam finds out who's behind the killings when he goes to search for her he finds her dead in the basement. As he searches the body for clues he doesn't find anything but he takes pictures with his phone and walks out to his car, as he gets in his car he notices the Impala parked behind the school and notices Dean getting into it and driving off so he follows him.
Theory 2. (Sam meets Deamon during the 200th episode)
Same thing with the theory above but when Sam arrives at the school he sees the Impala parked and goes into the school as he thinks he was seeing things. He talks to one of the teachers who knew the teacher that vanished and she says how many agents is the FBI going to send we already have 1 here. Sam gets a puzzled look on his face and notices Dean talking to a cheerleader not really caring about the case since he's still a demon. Sam slowly takes out his demonic handcuffs and walks over to him about to cuff him but Dean turns around and talks normally to him but his eyes go black. Sam not wanting to make a scene infront of everyone puts his cuffs away and asks Dean why he's here. Dean would shrug and say "School full of women Sammy can't resist" Sam threatens Dean telling him your still my brother and I need help with this case but I'm watching your ever move so if you do anything wrong I will shoot you. Through out the episode they would work the case and after they defeat the God who was behind it then they would leave the school. (2 more theories for this) They would either have a small fight outside because Dean is still a demon and he would knock Sam out and leave the scene making him have to search for Dean again or during the episode Dean would trick Sam somehow into thinking he's no longer a demon by maybe hiding his inner demon so the holy water doesn't work or get's the God they beat to some how announce that Dean has gone dark but some how got rid of all his darkness and is human again. So Sam and Dean would go in the Impala at the end with Dean driving it and as the camera zooms into him he would face the camera and make his eyes go black with a small smirk on his face. (This would lead to the season finale where Dean would act strangely by either not eating or he would keep scratching his arm with the Mark on it since it needs to be fed and he probably hasn't fed in awhile. Sam starts to notice this and asks him what's going on, but Dean plays it cool trying to hide his Demon side. So Sam pours some holy water into a drink of whiskey for Dean and passes him it Dean not knowing what's in it takes a sip and his mouth and throat start to burn making Sam know he's still a demon as Dean rushes out of the room Sam follows him. (This would lead into a similar scene to what happened in Soul Survivor the 3rd episode of this season) Dean would be hiding in a room waiting for Sam to show up but Dean would start to roar like he did while he was being cured in the 3rd episode making Sam hear him. (It still puzzles me on what that roar Dean was doing was about) So Dean would decide to show himself but their would be a knife in his back pocket as Sam approaches him Dean slowly reaches for his knife but Castiel would grip him from the back like he did in the 3rd episode making him roar once again. This would lead into the room in the bunker where Dean is still awake and Sam is about to cure him. As Sam starts to inject him with his blood that's purified Dean roars again but slowly starts to loose his Demon side so he gets knocked out from the injection, after he wakes up Dean is normal and looks around the room and says "You look worried fellas" He's about to smile knowing he's alive when black smoke comes out from the Mark. (I'm not sure if it would take the form of Dean it would be his dark demon side but worse or if it just goes through a crack in the wall making Dean, Sam and Castiel freaked out a bit but since Dean still has the Mark it would still effect him like it's doing now but Dean having to deal with the Mark effecting him would be the story for Season 11)