Being a teacher is a lot of work. You have to navigate the unique personalities and learning styles of a crop of students, manage lesson plans and grading, all while making education appealing to restless youngsters.
To add to this load, teachers are consistently underpaid, and are often expected to raise and nurture kids far outside the bounds of education. Whether consciously or subconsciously, neglectful and stressed out parents often don't consider how their lack of parenting affects a teacher's job.
If parents regularly start a kid's day by pouring stress and negativity onto them, it's only natural that the child will enter class feeling droopy and unable to focus on the lesson.