Melissa Mac, 15, is a year 11 pupil at Northampton Academy in Wellingborough Road but the school said the pupil, who is expected to sit her GCSEs this year, has not been to lessons since last July.
Melissa, who is of Chinese/Vietnamese origin and is 5ft 4ins, was last seen on Saturday, February 16, and police are concerned for her welfare because of her age.
Northamptonshire Police have appealed for the girl, who has not been in contact with her family since going missing, to let them know she is safe and well.
A spokesman for Northampton Academy said: "We have a schools liaison officer who has been involved with this the whole time.
"It's a very unusual situation and the school is concerned but the appropriate social workers are involved, and it's out of the school's hands.
"I haven't spoken to any of her friends but people are aware that she hasn't been in school since the last academic year.
"It's difficult because we're all concerned for her but there is only so much you can do and these are obviously unique circumstances."
Anyone with any information can contact Northamptonshire police on 08453 700 700.
Do you know Melissa and are concerned about her whereabouts? Contact the Chron newsdesk by ringing 01604 467030 or click here to send an email