The classes are not online, you take them in a classroom and sometimes go out of the classroom to explore certain aspects of your class firsthand maybe in the parks or hotels or whatever seems relevant. The only classes you can take online are ones that are from your college if you need to take them to keep up with your credits in order to graduate with your degree on time. I’m taking Exploring Marketing and Exploring Guest Services. Those two don’t count for college credit for me, but I’m taking them because it’s an amazing way not only to learn, but to network with important people which will help you with your career in the future with Disney.
Honestly there’s really not much in high school that can benefit you for this program besides doing research about it and knowing about what you want to say during the interviews and such. My high school was super small and offered the bare minimum class-wise. The only thing I would recommend is taking AP classes to get college credit ahead of time, that way you won’t miss much when you go to the program if you get accepted. I had almost a full semester worth of credits done before entering college because of AP, which (along with summer college classes) is allowing me to take a full semester off without missing anything or getting behind.
You don’t even enter your GPA for the application, it has nothing to do with the program. What matter is you nail your interview. I explained about the application more here, and my experience with the phone interview here.
I hope this helps!