The girl told Southport police, after the incident on September 14 last year, that two boys made sexual references before assaulting her.
She said: "I was angry and frightened and felt dirty."
The boy was convicted under the Sexual Offences Act.
Magistrates at North Sefton youth court proposed an action plan order for three months, with the boy having to report weekly to a Youth Offending Team member.
During the order he will be made to carry out a range of activities under the supervision of a youth offending team member to help him stay out of trouble in the future.