Just out of curiosity, why would you prefer a German school to the DoDEA school?
Also, how long are you guys planning to stay in that location? Do you want to do the German Abitur or get a US diploma? From what I’ve heard, it’s definitely possible to transfer, as long as it’s before you reach the Oberstufe (11th-13th grade).
I actually think it could be a great idea to attend a German school, especially early in high school. From my experience, unless you actively try to integrate yourself in German culture, you’ll be very separate from it on the base. I was born in Germany and lived there twice as a child, my parents were stationed at two air bases, and yet I knew practically no German until a couple years ago when I actively decided to learn it. I would say make the most of it—starting now in a Gymnasium will be very difficult, but you’ll learn a lot from it. Not only the language, but also the culture. I’m currently attending a Gymnasium in Bavaria and I find it very interesting how they teach subjects like history, geography, and religion. It’ll give you a completely new perspective.
Another note: your performance in the Gymnasium doesn’t really matter until you reach the Oberstufe, as long as you pass all your classes. In the U.S., how you perform dictates what classes/level of classes you can take in the coming years. In German schools, pretty much everyone takes the same courses. I guess you could see this as good or bad.
Definitely have a plan for graduation. If you’re planning to go back to US high school after 1-2 years, make sure you talk with someone (i.e. guidance counselor) about fulfilling your graduation requirements if you don’t want to repeat a year or take extra classes.