The children's charity NSPCC says nearly 19,000 children were admitted to hospital after harming themselves last year - a 14% rise over three years.
It called the figures "frightening".
And it said they should be a "wake-up call" to all those who care about the wellbeing of young people.
Sophie Allen, who is now 32, started self-harming at the age of about 11 or 12.
She said she "stumbled" across self-harm after accidentally hurting herself at a time when she was feeling particularly low after a lot of transitions and change in her life, and being bullied.