Japanese movie confession: Who is texting?

The person who sent the text message was Yuko Moriguchi in the play. "Confession" is a 2010 Japanese film adapted from the novel "Confession". Directed by Tetsuya Nakajima and starring actress Takako Matsu. The movie tells the story of Yuko Moriguchi who discovers that her beloved daughter was murdered in the school swimming pool one day. Although the incident was deemed an accidental death, Moriguchi announced to the students that the prisoner was in the class and launched his own revenge. In 2011, the film won the Best Asian Film at the 30th Hong Kong Film Awards.