>>195109I don't know much about Babymetal or Sakura Gakuin, but looks like this .gif is from a ceremony where they graduate members of Sakura Gakuin.
From Wikipedia:
"Since the group's theme is that of an elementary and junior high school, when a member graduates from junior high, she leaves Sakura Gakuin as well, during a ceremony the group holds around the end of the Japanese school year in which these members "graduate". The group releases a video of this ceremony annually, as well as a yearly album."
This video is from 2014, and Yui graduates in 2015, so I think she's sad because her friends leave. Correct me if I'm wrong.
