The girl was pulled to the ground and attacked in Chelsham Road at 16:45 GMT on Friday.
Her attacker was described as white, aged between 16 and 18, 6ft tall, with brown hair, pale skin and a square-shaped face.
Insp Dan Gutierrez, from Surrey Police, urged parents to remind their children to keep safe when they are out an about.
A teenage girl was assaulted while she walked home from school in Warlingham.