Although Junior school children and younger can be short-sighted (myopic), the teen years are notoriously the age when the risk of becoming myopic peaks. it is estimated about 1/3 of British children are short-sighted. Asian children have a much greater incidence of myopia, possibly as high as 70% of Asian school children are thought to be myopic.
Children are born hyperopic (long-sighted) to a greater or lesser degree. During the first decade of life this hyeropia usually reduces (emmetropisation). So by senior school age the eye's focusing mechanism should, without any accomodative effort, form a clear image of distant objects at the back of the eye (on the retina). Blurring of this image is a trigger for the eye to refocus or accommodate. In the hyperopic eye this can improve things but in the myopic or short-sighted eye it just makes things worse.
When emmetropisation does not stop at 'normal' but continues, the eye in its relaxed state focuses light in front of the retina causing the teenager to have blurry distant vision. If the refractive error is less than about -1.50 D(minus one and a half diopters of myopia) in both eyes then the teen can read a book quite comfortably without specs but will be starting to sit uncomfortably close to a computer screen. He or she will struggle with school work and may not even recognise friends across the road.
Below -1.50 D a myope may chose to only wear their glasses for school and for TV, but above this for social and academic development it may be better to wear the glasses all the time. This does not make the eyes lazy, it merely gives clear comfortable vision at all times. Alternatively the teenager may want to try contact lenses, especially if they play a lot of sports or are concerned about being bullied. Most teens can comfortably and safely wear contact lenses if they are motivated and supported.