The incident happened at around 8.10am on October 16, in an alley running along the south stretch of Maple Drive, near the junction with Packham Way, in Burgess Hill.
The girl was approached by a man not known to her and threatened, before she ran away.
Police said the man was in his mid-20s with blue eyes, stubble, and a leather jacket worn over a white hooded top.
The girl, who cannot be named for legal reasons, is receiving support from officers, and there will be an increased police presence in the area.
Detective Chief Inspector Pauline Lane said: “This was a distressing incident for the victim on her way to school, which we are taking very seriously.
“We are urging anyone who saw something suspicious in the area to come forward.
“This includes anyone in the area with relevant CCTV, doorbell, dashcam or mobile phone footage.
“Officers are investigating and will be at the scene as we carry out enquiries, and there will be increased patrols to provide reassurance.”
Witnesses or anyone with information can report it to Sussex Police online, quoting serial 343 of 16/10.