Often times in school, students are only well behaved in front of a teacher. On the first day, when students smell a perfume which is a neutral stimulus, they have no response because they do not know whose perfume it is.
Before conditioning, the unconditioned stimulus of the thought of the vanilla perfume causes an unconditioned response for the students to have thoughts of the vanilla perfume. As the students undergo conditioning, where the teacher walks in and the students realize that the smell came from the teacher and the perfume which is a neutral stimulus will elicit a natural reflex.
After conditioning, the students perceive the perfume as a conditioned stimulus where they have a conditioned response to be well-behaved because they know there teacher is coming into the classroom.