I can see what you're saying, I'll give you an example.

Classes are graded, right? So the top class has the best performing students in it, the lowest has the students that have trouble. So lets say I'm teaching the lowest biology class. A lot of concepts, even in high school biology, are a bit difficult to grasp and it requires effort and ability on the student's part for them to understand it and for them to remember it. Some of the kids in the lower class are just not the brightest (I know it sounds harsh), and biology is clearly not their thing. Other kids have had their parents force them to do this subject for whatever reason. The rest chose to do biology but didn't have a good idea of what was involved in the course, and later found they lost interest in it.

So then it's my job (sometimes) to attempt to teach these students who don't want to be there, don't want to learn, and can't grasp the content because they don't have it in them.

Compare this to the top class, who all have motivation to succeed in the subject, they do their work when they're told, they ask questions if they don't understand something, and soon enough they get it.

I mainly had to teach the lower classes though.

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