Romance one-shot about an emotionless girl
The manga is Ningyo Hime to Ouji-sama, by KYOUMACHI Hisa.

Synopsis from Baka Updates:


  School life becomes too much for Sonohara Kokone when Sei-kun, who is as popular as a real prince at school, suddenly asks her out. They barely know each other due to her doll-like but expressionless features. So, why is he minding her so much!?


this manga is correct because I read it ages ago and remember it. :|