1. Is the question wrong? The meaning of the question has nothing to do with the school. If you ask the number of students in the whole grade, the solution is as follows:
Let the number of students in the whole grade be x, then:
20% X * 2/3+20% X * 5/6-20% X = 2 1
The solution is X=2 10.
That is, 2 10 students in the whole grade.