chris1234,

On the question of how to deal with subjects like history where there is not so much repetition, you can always find out what time period and major concepts are going to be covered in the skipped year and do independent reading during the current school year or over the summer to cover that span and those topics. Honestly, 6th grade history is not likely to be so intense or in-depth that it couldn't be covered in a few weeks of dedicated study by a gifted kid.