According to the article, "Most cases of hearing loss are slight, affecting only one ear and involving mostly high-frequency sounds [...] About one in 20 have "mild or worsening" hearing loss, which can make them struggle to follow conversations or teachers at school."
Interestingly, the study doesn't seem to correlate listening to loud music with the hearing loss - they conducted interviews with several thousand kids and found that kids who listened to loud music for five hours or more a week were not more likely to have hearing loss. Other studies, however, have linked portable music players to a 70% increased risk of loss. Poor health in general can also contribute to hearing loss.
Teens may have a tendency to ignore their parents and think of themselves as invincible, but I remember going through school with a hearing loss. My loss was recognized and I have hearing aids, but I remember feeling stupid for not being able to follow directions or missing out on conversations. I didn't like feeling that way even when I knew I could blame my difficulty hearing. The problems kids might face from a slight hearing loss in school now, especially an unrecognized one, could magnify exponentially by college and by the time they enter the workforce.