Larger Class Size
We know that high schools have higher enrollments than middle schools, so it stands to reason that the individual classes are going to get bigger, too. Student-to-teacher ratios vary widely across the country, but in general, high school classes are larger than middle school classes. It might not be much, but it does make a difference. The jump from a 15-student class to one with 20 kids doesn't seem like that huge of a deal, but it does result in things feeling a little more impersonal and less one-on-one interaction with the teacher. There's definitely a lot less hand-holding in high school -- older kids are expected to be more responsible and independent, so they (in theory) don't need as much attention from their teachers. And a kid who's not quite ready for the task can end up falling through the cracks.