I know that I wish someone had given me direct guidance in how to handle credits while overseas. I would assume that a big deterrent for people considering a year abroad would be the fact your school may not accept credits. So I'll share what I did, and maybe that will give you some ideas of what you could do.
First step: Stake a claim in the guidance office. I practically LIVED there this year trying to figure out all my Germany stuff. Not only do you need their help on your application, but you'll need their help with credits. I would find the most liberal and probably the youngest guidance counselor you have and go the extra mile to get on their good side. I was really tight with my GC and it seriously pays off.
Second Step: Find out in general what classes you will be taking in Germany. This can be done by contacting former exchange students or asking AFS. Then find out how many credits you already have in school and how many you need for the coming year. If you have taken AP classes or an extra course, this will help you a lot because you wont need as many credits. Take this to your guidance counselor and ask him/her what credits the school is willing to give you for your school year in Germany. Important: Most schools will give you credit for math and science in Germany because it's pretty much fact that their standards are higher there. As for English, you will be taking ESL and therefore will not get credit for that and history will depend on your year in high school. Since I was supposed to take American history, I will not get credit for history in Germany.
Third step: Once you know what classes you wont get credit for, find a good summer school program. I highly suggest doing Summer school (even though it TOTALLY sucks, I'm not ganna lie) as opposed to some kids who do it during their year in Germany. Doing it during the summer gets it out of the way so you can enjoy your year abroad. I would think that taking it during the year would greatly take away from your experience plus your german. Oh yeah, and summer school. This is another thing that guidance will help you with. I did it through a program called Aventa. Important: Make sure that your summer program is the FULL COURSE and not the remedial course. I almost made this mistake and it could have cost me a lot. Also, summer school does cost a fair bit of money. I think each course was about $1000 dollars.
As a sidenote, for those who are told by their guidance counselors that they do not need the credits or that they will work it out when they get back, I would suggest getting a second opinion. The last thing you want to happen is to get back and realize that you should have taken Summer school. Some kids are taking courses just in case, which I think is smart but I would suggest figuring out exactly what credits you need so your not doing too little work but your not making up credits you dont need either.
So thats my two cents on making up credits. The only reason I post it is cause I got so frustrated trying to figure out where to start with making up what I need to for my year away. I am sure others are doing it differently, but I feel comftorble leaving with the credits I've made up.