If a student cannot see the blackboard distance to her she is suffering from Myopia. Hence doctor advises a concave lens of suitable focal length.
Myopia is the inability of an eye in viewing long distant objects. The image in this case is formed before the retina. For every myopic eye, there exists a far point beyond which a clear image cannot be seen. The short-sightedness is corrected by using a concave lens which diverges and shifts the image to the retina.