Give an example of a way to choose a sample of students that is poor practice because it depends on voluntary response

I assume that you are talking about something to do with opinion polling or some other thing like that and random sampling.

So, one way that this could happen would be if, for example, you were trying to find out how smart the average student is.  If you asked "who would like to take an intelligence test" you would likely get a skewed sample because of the selection effect.  Most likely, the students who chose to take the test would be the smartest students and your numbers would be higher than they ought to be.

I hope this is what you are asking.