Police said the girl was walking south in the 1500 block of San Anselmo Avenue at about 3:05 p.m. when a man driving a black van approached her and asked if she wanted a ride.
The girl declined and continued walking south along the roadway. By the time she turned around, the van had left.
Police described the man as white and in his mid-30s or early 40s, with a stocky build and short hair. He was wearing black plastic glasses.
He was driving a black van with two additional windows on the passenger side of the vehicle, police said.
Residents who notice suspicious persons or vehicles near a school are urged to call the San Anselmo Police Department at (415) 258-4610.