I'm not sure anyone here will be able to speak about education in South Korea, but I feel very qualified to state that lack of sleep isn't going to help anything. It's especially not going to help you remember stuff. I don't know what your parents think about it, but it seems your sleep is affected but the excessive amount of school work, and maybe you could bring their attention to it in some shape or form. I mean, it doesn't matter how many hours of science/math you pile on if you don't remember any of it... And also it's generally recommended to leave a couple hours of idleness before bed to let the brain wind down...