"But it was a complete misunderstanding," Ella says.
At first, the girls began calling her names, giving her the odd nudge and tormenting her at school. Then "it turned a bit ugly".
The group set up a Facebook page to attack Ella, posting pictures, nasty comments and rumours and encouraging others to do the same. She also started to receive threatening text messages.
"They would post pictures of me and tag me in them and get people to write things on them. It was like a Turn-Against-Ella page.
"They would call me names and use things against me. I got Facebook chats from people I didn't know saying, 'I can't believe you said that about me. I don't even talk to you or know you'. It got a bit scary at school."
Ella's experiences of bullying are shared by an increasing number of children as the rise of social media and cellphones changes the way children can be tormented.
The Herald's series on bullying aims to raise awareness of what children now face, and what can be done to stop it.
Netsafe executive director Martin Cocker says aspects of cyberbullying made it worse than "normal bullying". Every time a video of an attack was played, for example, the victim was victimised again.
Ella hid her plight from teachers and her parents for about five months, then decided to take things up with a school dean.
But things got worse when one of the girls in the group saw her leaving the dean's office.
"All hell broke loose then. When I got to class, [the main girl] started screaming at me."
She went after Ella, trying to hit her. The teacher could not control the girl and Ella had to be locked in a room while the girl continued to torment her from outside.
"She kept screaming at me to come out."
The girl was expelled for that incident and other girls in the group left school soon after.
The Facebook page was removed and the threatening text messages stopped.
But the bullying had a lasting effect on Ella. "Last year I went through quite a lot of depression and there was an older girl who really helped me a lot."
The older girl was a buddy for Kidsline and encouraged Ella to use her bullying experience for good - by joining Kidsline to help others going through what she went through.
She now deals with dozens of phone calls from youngsters, many of whom are suffering from the effects of bullying.
Ella, who is now 16, has a simple piece of advice for others being bullied: Speak up.
"Definitely talk about it to someone. I was strong enough at the time, but sometimes you need more help."
