Question

It is not the teachers, but the director of the school who ___ on the schedule

A. decide
B. Decides

( subject verb agreement)

Answer 1

A. because it's not proper

Answer 2

The answer is B.

The subject is not the teachers, but instead the director of the school; the director is one person, so you would use the same verb that you would use if you were just talking about the director.

for example, you would say that the teachers decide on the schedule, but you would not say that the director decide on the schedule. when referring to the director, the word decides would be used.