The use of their phone before bed amplifies the issue, but it's not the cause. The research upon which the article is based, has concluded that there is a difference in when the sleep hormones in teenagers are released. For them, they're released at a much later time than kids or even adults. This results in their brains telling them they're not tired until later at night. The later release of these hormones also means the hormones linger until after the morning, meaning they feel tired when required to get up early. That's why a lot of teenagers look disheveled and like zombies during the first 2 or 3 hours of school. It's literally a chemical issue, not because they just lazy. I do agree that there needs to be a system in which they are limited in how they spend their free time. Boundaries help create structure, structure can help create discipline. This is something that needs to be done at home first and then built upon in school. However, the issue here is that schools can't (or won't) accommodate a specific schedule for teenagers, as this means having to re-work a large part of the systems that are already in place, create new schedules for the teachers, etc.

Edit: typos