Here is what I done with my niece.
First, I explained the GTD system, and showed her the way I have mine setup. She already knows that I am very organized, and very productive.
I then explained to her that the reason I would like her to learn this is so she can stay up to date and current with her studies in school, AND still have the time to have fun and hang out with her friends.
I think this part is really important. Show her that she can have her cake and eat it to. Explain how most kids either study too hard and have no fun, or have alot of fun but get nowhere in school. But with GTD, she can have both.
And explain to her that by using GTD she will actually spend less time on homework, than if she didn't use GTD. The reason is because she will be quickly figuring out projects, goals, actions, and having a good system will allow her to move quickly through her actions and complete everything, allowing for more time for fun and socializing.
I also had her:
A) treat each class as a different "area of focus and responsibility"
B) treat each homework assignment as a separate project
My niece has told me that this has really helped her have a clear picture of how her classes fit together. She really likes the areas of focus and responsibility. And when she started part time job, she was able to quickly add that to her list with no problems.
Two more ideas.
if you are getting her a cell phone, then one thing that you can do is show her how to use the calendar properly, specifically for setting alarms/reminders about things that she needs to do, and due dates for her homework assignments.
And secondly you could look into having her buy an app that will help her manage her action lists and projects.
Actually another good idea is set her up with a Free GTD trial account and have her watch the "Paper Planning Model" video. This would be great if she chooses to manage everything by paper.