You don't major in anything in high school; that's something you do in college. In 11th grade, some kids take AP (Advanced Placement) classes, so that's what I would probably use.

I'm not sure about your second question, but I think just giving the GPA should probably suffice.

Translating grades is difficult to impossible but you can go to a source here
http://www.foreigncredits.com/Resources/GPA-C...
and try your best.
To follow up on dude's comment, an AP class is a rigorous, college-level class taught in high school for advanced placement in college. Some colleges then allow to skip this class. The curriculum is set by the College Board to a nationwide standard. Your Vertiefungsfächer are not likely to have followed these guidelines. Classes that do not follow the AP guidelines, but still are advanced, are called honors classes (in the American school system).

Remedial courses have a different meaning in BE -
http://www.oxforddictionaries.com/definition/...
Provided or intended for children with learning difficulties