Sirius was the first to say it. Remus had been going on about the other Marauders needing to do the potions homework all day. In the end Sirius had enough of the nagging and gave in. "Yes professor Lupin I shall do the homework" Sirius said voice dripping with sarcasm. The others had laughed but did their homework on time.
Another time Remus was warning James and Sirius not to do a prank because they all knew it would lead to detention. After hearing about their plan failing all day James finally agreed. "Okay professor, no prank." Sirius spent the rest of the evening pouting at Remus.
When Peter called Remus professor it involved chocolate. "You shouldn't eat food like that before an exam" Remus had said gesturing to the pile of chocolate at the bottom of Peter's bed. "It will make you feel tired and sick" Remus continued.
"So" Peter said shrugging his shoulders. "So you could fail the exam" Remus replied seriously. "Okay professor" Peter said as he stood up and handed the chocolate to Remus. When Peter left the room Remus smirked at Sirius and took a large bite of the chocolate.
Over the years Remus made more comments leading to the other Marauders calling him professor more and more.
One year for Remus birthday they decided to all put some money in to buy him a small case with Professor R. J. Lupin stamped across the corner. Obviously they filled it with chocolate - "it wouldn't be moonys birthday without chocolate" Sirius had said dumping a large pile into the case. When Remus had opened it they all laughed and spent the rest of the night sharing the chocolate and laughing.
Remus had gone up to bed before the other boys that night and took his new case with him. Sirius had though something was off, Remus looked sad when he left even though he had been laughing with the others all night. Sirius excused himself and followed Remus up to their dorm.
Remus had been crying on his bed staring at the case, suddenly it hit Sirius. Remus could never be a professor no matter how much he wanted to be.
Sirius walked over to the Sandy hairs boys' bed and sat down beside him. Neither of them said anything, they just sat there, at some point Remus leaned on Sirius and he let him. When the other two came in they didn't say anything Sirius just nodded at them to say it was alright and they went to bed.
After that birthday Remus took his case with him everywhere every year before school he would pack his things in it and put it on the shelf above his head on the Hogwarts express. They still called him professor when he was telling them what to do but no one ever mentioned what happened on his birthday.
Over the years of taking the case to Hogwarts and everywhere after Hogwarts: When they joined the order together, the case became battered and the letters began to peel. Remus thought that he would always use it to remember his time at school and then he lost everything. The night that James and Lilly were killed by Voldemort. Remus had lost his only friends, they had either been murdered or had become murderers.
It took less than 24 hours for everything that Remus cared about, everything that matted to him to be taken away. It almost killed Remus to find out what had happened. The first thing he did was shove the case that his friends had bought him to the back of a cupboard and try to forget about it. Of course the Marauders had all brought each other things over the years but the case was special to him. It represented everything that he could never be and everything that he wanted to have.
Years later when Remus is struggling to make a living Dumbledore shows up at his cottage and offers him the job he had always wanted. Remus was hesitant and Dumbledore had explained how it would work. Remus took the job secretly overjoyed, he didn't even care that the job was cursed and it would probably only last a year.
Remus pulled the case out of the cupboard and put all of his things in it just like he used to before when he was young and going to Hogwarts. The professor had to take the Hogwarts express, he couldn't afford another way of transport. He sat in the same compartment the Marauders always used to and fell asleep.