"Inspection paradox" seems like a misnomer. It has nothing to do with inspection and it isn't a paradox. It's just selection bias [1] or more specifically, sampling bias [2]. In the case of class sizes, are you asking the average class size from the point of view of a student (N per class) or a teacher (1 per class)? Take the extreme case where you have one class with 99 students and one class with one student. From the student's point of view, the average class has 98 students. From the teachers' point of view, the average class has 50.
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[1] https://en.wikipedia.org/wiki/Selection_bias
[2] https://en.wikipedia.org/wiki/Sampling_bias