Answer:
In the principle's mind:
Student is Tabula Rasa.
Teacher is there to fill the empty space.
Time goes by. Exams arrive.
Student does badly.
Teacher has failed to teach.
Normally this isn't a problem on its own as some students are just too dense/disinterested to be taught, but if multiple students are all doing badly, it's often (not always) a sign that the teacher cannot interest them enough in the subject for them to learn.
Especially in a private school, it's easier to kick out and replace one "failing" teacher than it is to kick out and replace multiple paying students(/customers.)