Actually, last year someone asked and received the answer that NON-homeschoolers could indeed (I love saying that word, "indeed!") join as AFTERschoolers and take advantage of the group buys. So go ahead and sign up, guilt free!
FWIW, DD 7 took the cyber-ed middle school physical science and life science last year. She has never before taken any formal science, but she breezed through it. She did not like the physics much, but she loved the bio.
DS at 10/11 took the high school level cyber-ed bio and chem and he especially loved the chemistry course.
These are self paced courses; the upper level have more lessons than the lower level. It was interesting to find out that my son qualified for the MIDDLE SCHOOL courses through his JHU/CTY talent search. Yikes! I am glad we didn't pay the big bucks for him to take courses that wouldn't have been remotely challenging.