Responses
I'm going for 2 and I'll have 1 as a side dish. I'm kinda having the idea that sensei is the main piece, though. He probably knew he was going to go away, and all the girls are some temporary distraction (I'm not even sure if he did anything with them other than hang out), but he always goes back to sensei.
But yeah, he's a secretive kid, do it could be anything.
Could either be one of two:
1.) A manipulative move to make sure that sensei keeps thinking about Nanao even when he's gone, to ensure loyalty. Not a new or unusual move, I've seen it before in BL as well as real life. Nanao may be a selfish playboy, planting the continuous loop of "He kissed me, omg he kissed me" in sensei's head.
2.) Nanao might genuinely be into sensei, as we've seen through his reliance on sensei's scarf as a "lucky charm". He could just be a manipulative type of seme who wishes to emotionally push sensei to the brink, or he's not good in dealing with feelings. However, the fact that he uses sensei as a side piece while he dates girls sets off red flags.
What do you all think? I agree with a few other comments about Nanao giving off an unsettling feeling.