20 people / 100% * 5% = 1 - this is a leader. Increasing the number of people entails a loss of management.
In the classroom with 30-40 persons, it will be very difficult for the teacher to set the tone of the class and constantly keep the attention of the group.

As a public school teacher I can confirm that this is 100% correct and I came up with the same numbers years ago. Of course we need to make sure this is not used against us like in this case