Well, of course I don't have all the answers. I can only surmise what's happening. I do know where I live it's no big deal to put the kids back into school at anytime. I believe you just tell them that you would like to quit home schooling and then they set up a meeting. The child is questioned and you bring his work with you to show where he is at. I believe that's all there is too it. There might be a disagreement about which grade he or she would be placed in but that could be arbitrated if it came about. I think that would be a rare case. Usually I believe the child is placed into the grade that he belongs in with little difficulty. By law the school system must accept the child. I don't know what would happen if you decided to put your child back into school in 12 grade with 6 months to go. It would be interesting to see where he would be placed. If the school wanted him or her to start 12 grade over again I believe it would be up the the school to prove that the child isn't up to the level of the rest of the 12th grade students. If the child took a standardized test and was up to par I don't believe there would be anything the school could do but let him finish out the rest of 12th grade and graduate him.
I would imagine if the child was home schooled since kindergarten and you then put him or her into high school it would be a bit of a culture shock.