We had friends who pulled their children out of school (middle school and high school age) and did a sailing trip around the world for over a year. They re-enrolled their children back into school when they returned.
With high school classes, I would think your children would be placed into appropriate classes upon re-enrollment and would continue in high school until they had enough credit to graduate (so if they still had two years of credits necessary, they would spend two more years after re-enrollment).
Thinking outside the box a bit is usually less complicated in these situations than we think it might be.