The `X Factor` judge - who was bullied when she was at school - is the new ambassador for Secret`s Mean Stinks campaign, which encourages girls to "gang up for good" to end bullying in schools.

She said: "I heard about the programme last year, and thought it was such a great opportunity. They`re really trying to put an end to bullying."

The programme asks girls to pledge to have a "Drama Free School Year" by making a pinky swear, symbolised by wearing blue nail polish on their pinky fingers and Demi added: "I`ll definitely be wearing [the nail polish] starting soon. It`s a conversation starter: `Hey, why is your pinky blue?` `This is a pinky promise that I`m not going to bully people, that me and my friends are ganging up for good.` "

Demi, 20, also spoke of the tough time she had at school because she was targeted by bullies.

The singer - who spent three months in rehab for her eating disorder and self-harming at the end of 2010 - told PEOPLE: "I had a really tough time when I was in middle school. People would write `hate petitions` [about me] and send them around to be signed. They`d have CD-bashing parties of my demos. They`d come to my house, stand across the street and yell things. It was a very emotional time for me, and all I wanted to do was get away."
sections: Celebrities