A course that provides day by day consistency forms habit and momentum, and that momentum gets you through the times when you feel like fizzling out. When you are tired, having to make the decision to study, having to decide on a new topic or what to revise can stop you completely through choice paralysis. It is in these moments that the momentum of an online course can get you through.
Schools do an amazing job in continually challenging circumstances. But let’s face it, outside of specialised schools; they cater to your class, not to you. An online course allows you to take your time to complete an exercise, corrects it immediately providing important feedback, and allows you to come back when it suits.
If the level in your class at school is quite mixed, or if there are disruptive students, the challenge you need to be excellent; the motivation from other students with expectations of mastery, will have to be found elsewhere. Most of us see daily that teenagers need peer approval to be motivated. It is critical that they are surrounded by like-minded people. My personal experience is that teenagers are careful not to grow (or be seen to grow) in a direction that makes them seem different. Even excellent students who are otherwise motivated make a point of doing no more work than their classmates.
However being online means you can join like minded people, and when teenagers are interacting with other students who are facing the challenge of working hard, they can increase the work they are doing literally tenfold.