Notice that most of the Coal Hill students call Clara by Miss. Rarely ever Miss Oswald, just Miss. Which would be fine if so much focus wasn't put on it in a series whose main (presumed) villain is Missy. The trailer for Dark Water seems to be suggesting that Clara is not as she seems, which with this seems to suggest a link between the companion and the villain. If the show does go all out and presents a massive twist it would be awesome, and to some degree hinted at (if not set up) by this verbal trick. There has, in fact, already been on potential confusion between Miss and Missy, seen in the most recent episode, In The Forest Of The Night. Maebh, one of Clara's schoolchildren, goes to find the Doctor in the TARDIS because Miss told her, despite Clara not having yet realised anything was up. This is quickly brushed under the table by saying that Maebh probably heard Clara's thoughts and took it as a conscious command. But thinking out this, it doesn't really work, feeling too specific a sentiment to be throwaway. Could it have been Missy, who we know was observing the events of this episode, trying to influence events. It wouldn't be the first time. Conclusion: This is probably a red herring to help boost the sense that Clara will betray the Doctor to some degree (evidenced in the trailers), but it could be a hint to something much more. Heck, it's not the first odd link between Clara and Missy's characters...
