The principal kept the staff members waiting.

Explanation
The given sentence is in the active voice, as the subject 'The principal' performs an action 'kept' against the object 'the staff members'.If this sentence is changed to passive voice, then the subject is also changed, in order to retain the meaning of the sentence. So, the object takes the place of the subject and vice versa and the word 'by' is introduced as now the subject receives the action by the object. The verb form of the sentence is also changed from active form to passive.