How competent the parent is versus how competent the teacher is, how big the respective "classes" are, etc.
School can be great for knocking the rough social edges off kids. Can also be a very inefficient use of kids' time. Homeschool can be great, but sometimes you have incompetent parents who just want to shield their kids from the liberal agenda or whatever — and sometimes it can even be a way of shielding abuse from discovery.