Students will probably say that the students in this class would not be representative of all the students in the school. The students in the class would be a lot older than those in lower grades or younger than students 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. Students will say that the sample is probably too small to be representative of the whole population. One of the things you need to consider when picking a sample is the size of the sample.
Statisticians have sophisticated ways of selecting sample size.
One general rule they use is to pick a sample equal to about one tenth of the total number in the total population. For example, if the size of the population is 1,200, then we would select a sample of about 120. If we select 65 students at random, we would have a good sample from which to make some predictions. If we picked more students, it would make the predictions even more reliable. To see if they have support for such an addition, they want to survey voters in their school district.