Bias in samples and surveys

A high school has a policy that students' phones must be kept away during class. A principal used the school roster to poll a random sample of

50

students, and only

10 %

said that they ever had their phone out during class. The next day, the principal observed classrooms and noticed that approximately

25 %

of students had their phone out at some point during class.

Which of these is the most concerning potential source of bias in the principal's poll?

AP®︎/College Statistics