The boy, from Lincolnshire, groomed some of the girls on Facebook and encouraged them to send him photos of their breasts.
He was aged between 14 and 16 when he committed the 10 offences over a 17-month period.
His victims were five girls aged between 12 and 15.
Sentencing the boy, who cannot be named, Judge Michael Heath said he regarded him as "dangerous" but would not impose an extended sentence because of his age.
The teenager was made the subject of a sexual offences prevention order and must also register as a sex offender.
Judge Heath told him: "You targeted and groomed girls, and indulged in persistent pestering of these girls because of their age, by social media and other means."
The boy admitted the 10 offences, which included three charges of rape.
Grace Hale, prosecuting, said two of the girls were encouraged to perform a sexual act on the boy while another had underage sex with him.
In a statement read out at Lincoln Crown Court, one of the victims described how afterwards she felt "horrible, used, scared and worried."
In mitigation, Caroline Bradley said the boy had been badly bullied by his peers at school and found it easier to mix with younger children.
Miss Bradley told the court he was a bright boy who worked hard within his family and understood what he had done was wrong.
A 16-year-old boy who raped and sexually assaulted younger girls has been sentenced to eight years' detention.