Children who have been educated at home generally can transfer back to a classroom without serious difficulty. Generally speaking, school staff can and do review the kind and quality of a child's progress, and administer standardized tests, and thus determine the appropriate grade placement in their system. Achievement tests and portfolios can be used to validate secondary credits.
Children who homeschool may study different topics than those being covered in the local school district, but this often is the case if a child transferred between school districts. Some school districts operate according to a far different philosophy about at what level in a particular subject a child of a certain age will be instructed.
Children who transfer from home school into a classroom need some time to adjust to getting most of their information from lectures and moving along at the same pace as the remainder of the class.