The students in the class are a lot older than those in lower grades or younger than those in higher grades. Picking this class as the sample would create a biased sample, because it would not be representative of the school as a whole. Discuss with students the meaning of the word biased. Students should say that the sample is probably too small to be representative of the school's population. One of the things you need to consider when selecting a sample is the size of the sample.
Statisticians have sophisticated ways of selecting sample size that depend on what is being tested.
In some situations they may use a sample equal to about one-tenth the total population. Since it is easy to find one-tenth of a number, we can use this approach for this activity. For example, if the population is 1,200, then our sample should be about 120. If we selected 65 students at random, we would have a good sample from which to make some predictions. If we selected more students, it would make the predictions even more reliable.