Police said witnesses, including Rebecca's ex-boyfriend, told them she did not engage her tormentor, leading them to believe she was being bullied.
The 14-year-old suspect also allegedly bullied anyone who was friends with Rebecca, according to police, and even encouraged the 12-year-old suspect, who was Rebecca's friend, to turn on her.
The younger girl was suspended for one fight in February 2013, according to police, in which the older suspect encouraged her to "beat up" Rebecca.
During a recent interview, police said the 12-year-old girl told them she bullied Rebecca and expressed remorse.
The 12-year-old was released into the custody of her parents and has been placed on home detention pending further proceedings.
Rebecca tried to get a fresh start at a new school for the 2013-14 school year, authorities said, but the torment continued online.
ABC News' Alyssa Newcomb contributed to this story.