We sent our twins to b&m 8th grade to help us decide what to do for high school. That way, they could learn the school way without it affecting their transcript in high school. I do think that it might take a marking period to adjust to the requirements of school (showing work for each math problem (was a battle for here for years for ds), keeping the notebook organized, writing enough, etc...) - not really a whole marking period, but there is an adjustment period.
One of the deciding factors for me was attending the curriculum fair for upcoming 9th graders. (The school's version of a curriculum fair is different than a homeschooler's.) Teachers from different subjects spoke about what the students would be learning in 9th grade. Each teacher was passionate about the subject that they taught. I want my kids taught by someone who is passionate about the subject. I am not. So far, my twins teachers are passionate about their subjects and it comes across in their teaching. My kids like subjects at school that they probably wouldn't have enjoyed under my direction such as physics & chemistry.