A spokesman for Sussex Police said Iesha Manuk-Welsh was last seen leaving school in Hastings on Monday (21 October) at the end of the school day.
She had told a friend that she was meeting someone in Filsham Road and has not returned home since.
Iesha is white, 5' 5" with long brown hair. She was last seen wearing black jeans, a white shirt, a navy blue school blazer with turquoise piping and black coat with black and grey Nike trainers.
PC Jo Seabridge said: "We are concerned for Iesha and believe she may have had a bag containing other clothes with her so she may have changed. She has links to Surrey and Sutton."
If you have seen Iesha or have any information about her whereabouts, report online or ring 101 quoting serial 21/10.