Accept the students in your class as they are.
They are given to you with their limited abilities, background knowledge, skills, and training.
We often wish that they were smarter or better prepared.
If we don't accept students and work with them as they are there are several things that can happen:
1. We can waste a lot of emotional energy complaining about the quality of our students. "In the good old days .... " or "When I was a student ...."
2. Frustration with the students can create emotional distance (even hostility) that reduces the effectiveness of our teaching.
3. We plough on with our planned curriculum, level of course, and high standards. Most of the students then learn almost nothing. Our goal should be to move them one step further in their education, ... for them to just learn something.
For example, in third year physics it may be making sure that they finally learn what we think they should have learnt in first year [e.g. how to work with physical units].
This may require adjustment of our plans and expectations.
There may be legitimate issues about pre-requisites, admission standards, or high school curricula that need to be addressed to improve the quality of the students we teach. However, once the class starts all that is irrelevant. There is nothing we can do about it.
Those issues are to be dealt with in a different forum.