Hi Ania,
The BYU classes sound excellent and very affordable--I have looked through their catalog before for my guys. As delbows says, 0.5 credit refers to one semester, so that if Ghost has 3 high school science credits to graduate, and the HS accepts the course for credit, he would need 2.5 semesters more (shouldn't be a problem to get enough credits, but getting a HS to accept it may be--unless you ask for it to count toward the electives).
Would the teacher really let him work online in class? If she's already defensive, I'd worry if she'd really give him her support (time, space, computer, quiet). If you have to do it at home, it's scheduling. I find online courses a great option, but VERY stressful for me. 3 of my kids are doing online work now, and I'm way more worried than they are about their readings, homework, emails from their teachers, schedule, making sure the computer's not wicky-wacky, etc.
Also, Ghost is brilliant. Are you underestimating his ability by doing a standard physics course as opposed to AP physics that he could test for college credit at the end? I read an article in the Washington Post about an average 8th grader who took APEX Learning AP Biology. Suddenly he blossomed, scored a 5 on the test and is now blasting through other AP courses. Ghost is not average.
Even if you don't want to do AP at this point, I'd give the course a shot to supplement and then decide on options for next year (the first year teacher excuse doesn't sound promising for year 2).
