Answer:
4 students will transfer from School B to School A.
Explanation:
School A has 216 students and 12 classrooms.
Or 
 students per classroom
 students per classroom
School B has 104 students and 4 classrooms. 
Or 
 students per classroom
 students per classroom
Now, the average will be :

=> 
 students per classroom
 students per classroom
As school B has more than the average students, we will transfer 
 students to A, that makes 22 students per classroom.
 students to A, that makes 22 students per classroom.
Therefore, there are 4 students who will transfer from School B to School A for the ratios of students to classrooms at both schools to be the same, which is 22 students: 1 classroom.