As far as HS Graduation/GED... I think the missionary department just wants to know your current educational level to help assign you to a mission that is best for you.
For example: if you have a 4th grade reading level (and there are missionaries that have a low reading level), it would probably not be wise to send that missionary to a foreign language mission if they have a hard time with their native language.
I would ask your Bishop for his recommendation on how to complete that particular question. (I'm pretty sure the missionary department doesn't have the time to do an educational background check.)