The mother of missing teenager Clancy Ellis has made a heartfelt plea for her daughter, who was bullied on a social networking website, to return home.

Clancy, 15, disappeared on Wednesday morning after leaving Alstonville High School, inland from Ballina on NSW's far north coast. She left a note indicating she was travelling north and would not return for several months.

Mother Michelle Ellis, writing on the 'Help Find Clancy Ellis' Facebook page, said she feared someone was "following through with their own plans" for her "pure and gentle" daughter.

"Here's what I don't know," she wrote on Sunday morning. "Have you judged someone that you thought might be offering you refuge - a safe haven - a break as trustworthy and darling is that person who you thought?

"You can't imagine the scenarios that play repeatedly through my mind of what COULD have gone wrong with your plan."

It was only following her disappearance that the Ellis family discovered evidence of cyber-bullying carried out against Clancy, in particular on the website Ask.fm, which enables users to anonymously post comments to each other.

Mrs Ellis said the "awful" and "just gross" things written about her daughter, who suffered the online abuse in silence, were far off the mark.

"I know you are so so beautiful and sweet natured ... you got cranky at me because I squashed a mozzie on your arm and you tried to revive a fly that you accidentally killed," she wrote on the Facebook page. "You want to be understood and you want to be loved."

Clancy had been deeply unhappy in the months before she disappeared, plugged constantly into social media, and had stopped reading books, which had previously been her passion.

Clancy appears to have packed a bag with a few changes of clothes, her mobile phone and laptop and their chargers and a new paperback copy of The Hitchhiker's Guide to the Galaxy. Her family hold fears for her health and safety - she had been battling tonsillitis and a chest infection.

Clancy is 165 to 170cm and thin, with blue eyes, dyed red hair and a piercing in her right nostril. She was last seen wearing an Alstonville High School jumper.

"My baby darling sweet child please please please, if you can contact someone anyone to assure me," wrote Mrs Ellis. "I love you. PLEASE be safe."

Police are investigating, and have asked anybody with information on her disappearance to contact CrimeStoppers on 1800 333 000.