Being a high school student is difficult. Multiple classes throughout the day, some of which may be legitimately boring, would be strenuous for an average adult to sustain the attention necessary for success 5 days a week. However, if there is something your adolescent takes an interest in, I doubt they have significant difficulty in attending to these interesting and personally fulfilling/enjoyable activities. What I tell most families is that their child should, like most people, seek a career in something they find interesting. Once they get out of the broad general education of High School and begin to narrow their studies through college or learn a vocation they are interested in, it is very likely they may require less, and eventually possibly no ADHD medication to succeed.
The underlying neurobiological mechanism that manifests as ADHD shouldn't be expected to change much from adolescence to adulthood. However, some symptoms are more prone to spontaneous resolution than others. Such as hyperactivity and impulse control do tend to improve with age.