A/N: Just a tidbit of info: In this story, Voldy returns after a few years, per Dippet's suggestion, since he knows Dippet is his best bet back to Hogwarts. He gets the job, and...
1951
"Well, I'm certainly pleased you've agreed to hire me this time, Professor," Riddle said, smiling broadly as he exchanged a handshake with Armando Dippet.
"Believe me, Tom, the pleasure is all mine," the headmaster replied. "Welcome back to Hogwarts."
The office door opened, admitting a tall figure in flamboyant plum robes with graying auburn hair, a sheaf of papers in his hands.
"Armando, I've brought the last of the applications for..." Albus Dumbledore's voice trailed off as he registered Riddle's presence. "I'm sorry, I didn't realize you had company. Hello, Tom."
"Hello, Professor," Riddle said, unable to keep the corners of his mouth from tilting upwards despite his familiar feeling of annoyance around Dumbledore. "No interruption at all, I assure you."
Dippet turned, happily looking from one man to the other, oblivious to the reigned-in animosity from one and the benign suspicion from the other. "No need to give me more applications, Albus, I've made my decision," Dippet said smiling. "You're looking at our new Defense Against the Dark Arts teacher. Our youngest yet, I might add," he clarified.
"Indeed? Welcome to the staff, Tom," Dumbledore said. Riddle inclined his head, smile broadening. He was aware, however, of the irritating fact that Dumbledore used his name far more than was necessary. He was sure Dumbledore was conscious of this; the Legilimens had a disturbing knack for recognizing Riddle's thoughts. Something to take note of, but not unduly worrying, he thought.
With that, Riddle returned to his apartment in Knockturn Alley, and Hogwarts School of Witchcraft and Wizardry reverted back to the everyday happenings of the summer before school's reopening. Dormitories were cleaned, syllabi were constructed, and anticipatory children received fat parchment envelopes containing school supply lists and necessary information.
The new school year began as planned, with a welcoming speech from Dippet and a chorus of voices, the prepubescent, the embarrasingly warped, the matured, and those precious few melodious voices that stuck out among the din joined together in a most un-melodious manner in the Hogwarts School song. Then the moment struck when the Headmaster must introduce the new member of staff for the year.
"Welcome, all of you, to another year at Hogwarts," Dippet said, a benevolent smile in place and arms outstretched, as if he wished to personally welcome each and every one. "Before we begin the feast, I would like to introduce your new Defense Against the Dark Arts professor, who some of you older students may remember from your first year days. Please help me welcome Professor Tom Riddle."
The applause, though not really deafening, was enthusiastic enough, Riddle thought appraisingly. He cast his eyes around the room, briefly stopping upon familiar faces like Abraxas Malfoy's, now a seventh year, and the like. He rose. "Just because a fair amount of you seventh years may know me on a first name basis," he said, his charming facade firmly fixed in place, "I still expect you to address me as 'Professor'. Humor me." He sat as the applause increased in volume and intensity. He found this response most satisfactory to his overlarge ego. For the time being, this would do.
"Thank you, Tom," Dippet said, smiling fondly at the alum. "I have just one more thing to say. I have enjoyed serving as the headmaster of Hogwarts for half a century. It is with regret that I inform you of my impending retirement. I will not be returning next year." Whispers and dejected sighs flitted across the Great Hall.
"However, I will be succeeded by Professor Dumbledore, who I have every confidence in; he will surely uphold the traditions and the standards of excellence Hogwarts is so well known for." The applause resumed and Dumbledore politely raised a hand, eyes twinkling, as if to brush it away. He cast a stealthy glance in Riddle's direction, and witnessed, to no great surprise, a look of abject shock, which Riddle quickly returned to his usual impassive expression under scrutiny.
"Without further ado, let the feast begin!" Dippet announced, and the tables filled.
Apparently, Dippet retired no earlier than 1951 and no later than 1955, so I think it's safe to assume this would be his last year.