Give another example of a bad way to choose a sample that doesn't use voluntary response.

Another way that you can choose a sample badly (without asking for volunteers) is if you choose a sample that is likely to not be representative of your student body as a whole.

For example, let's say you are trying to determine the average height of students at your school.  If you decide to use the basketball and volleyball teams as your sample, you are likely to get an incorrect average.

Similarly, if you want to measure how much body fat the average student has, you will not get an accurate average if you choose to measure only the cross-country team.