My son was having to miss school a lot due to being sick. Even with a 504 plan in place it made it really difficult to keep up. Though the 504 plan is a great tool to have, we had it arranged so that he could do an abbreviated portion of the work. He could take quizzes and test then move on as long as he showed good comprehension.
Though we now home school him due to the amount of time he has spent in the hospital, which takes a lot of pressure off of him.
I have to agree with Twist of chaos, education is second to health. without health there will be no education.