I am a High School student going into 12th Grade who hasn't learned anything since 8th grade. What courses do you guys suggest I can do to catch up? In all subjects...

I suggest you starting with math 8th grade and mastering it. Whenever you finish do the Get Ready for Geometry as the Algebra I will be surely completed by the time you master the 8th grade course. Then master the Algebra I and Geometry courses. Then the Algebra II. Then you could continue with math (seeing Trig and then Precalculus) and at the same time do Chemistry and Bio. When you finish Precalculus you could the HS physics course and the AP one. Then you could master HS Statistics and AP Stats. By this time you could have finished all the sciences, except for the library in Physics and maybe Chemistry dunno about this last one. At the same time you can be doing all History and Econ from the beg as they do not need prereqs. 

Then you could do Calculus AB and BC and master them, complete the library of Physics and before all this you could have started with some compsci and programming. I would say to start compsci and programming when finishing precalc but you can start whenever you want. 

And there you would have a standard hs path. You could maybe do some Multivar Calc or Linear algebra if you want to. By that time you would have completed a good chunk of KA's content! 

For me the most difficult topics are compsci & programming, then chemistry and then bio. But it might depend on the person. Nothing's impossible! 

Thanks a lot