I was speaking from personal experience. I found that as a high school student, I did not have the focus or perseverance to learn most subjects on my own, and so having an actual, in-person instructor (combined with peer pressure) encouraged me to keep up with my work and study at least somewhat effectively. There are dynamics in a live classroom that cannot be replicated in an online classroom: the presence of your friends and peers, the ability to walk up to the teacher to ask for clarifications or assistance, etc.
As far as grading systems, my high school was A-F. Most of my teachers graded on a curve, with the goal of keeping the class average around a B-/C+. However, it's been at least a decade since I've been in a high school classroom, so I can't comment on current grading techniques. I don't know when schools are mandated to take Algebra - I can just tell you that I took it in 8th grade. No idea if that is normal or not.