Written for BonesOfBirdWings for the 2017 Trick or Treat Exchange on AO3

It started during Teddy's first year at Hogwarts. He wrote to Harry every week, and his letters quickly began filling with questions and requests for advice about schoolwork—in particular, about Defense Against the Dark Arts. Harry was happy to help, in between changing diapers and getting Albus to stop trying to eat the owl treats. And when Teddy came home for the holidays, Harry found himself giving practical demonstrations and mini-lessons. It was a little like Dumbledore's Army all over again, except they practiced in the back yard instead of the Room of Requirement, and the only goal was fun, and keeping Teddy's grades up.

Then, that summer, Minerva McGonagall came to visit, and offered Harry the Defense Against the Dark Arts teacher's position at Hogwarts, as the bloke who had filled the position since Voldemort defeat had decided to pursue a research position somewhere in South America.

"I know you've had your heart set on being an Auror since you were fifteen," she said, both of them pausing to remember that particular meeting, "but you also did a great deal of teaching that year, and I think we can all agree that you were very good at it."

Harry told her he would think about it, and the next day he woke up to half a dozen owls from various members of Dumbledore's Army, all explaining in great detail how he was perfect for the position. Hermione even came to lecture him in person.

The next thing Harry knew, he was turning in his resignation at the Ministry, promising Ginny he'd come home on weekends, and was scrambling for advice from McGonagall and Neville about putting together lesson plans, and grading essays. Merlin's beard, he was going to have to assign and grade essays.

Neville told him to think of it like the DA. "Only now you can take away house points when the students get cheeky," he said over ice cream in Diagon Alley while Harry tried to figure out what books he wanted to assign his students. "Or give them detention when they're really bad," Neville added, after a second's thought.

"Do you have to do that a lot? Give detention?"

"Not really. Only if they damage one of the more fragile plants. Mostly if they don't listen to me there's rather immediate consequences—so everyone's usually good about listening, especially when we get to the more dangerous stuff. But I don't think you'll have much problems with your students, at least in the beginning."

"Why's that?" Harry asked, remembering the first session of the DA.

"Well, you're older, for one thing," Neville said. "They expect you to know what you're talking about. And you were an Auror, and you're Harry Potter." He shrugged when Harry made a face. "You won't get anyone snickering about learning expelliarmus, is all I mean."

"I guess."

"Gran, look, it's Harry! Hi, Harry!" Teddy ran up, dodging some witches struggling with a crate full of what sounded like a particularly rambunctious cat. Andromeda followed at a more sedate pace, carrying some packages from the apothecary. Teddy was sporting bright green hair that afternoon, and a packet of chocolate frogs. "Are you really going to be our new Defense Against the Dark Arts teacher?" he demanded, skidding to a halt in front of their table. "Hello, Professor Longbottom!"

"Hi, Teddy."

"Yes, it's true," Harry said. "Hullo, Andromeda."

She smiled at him. "Hello, Harry. Looking forward to the school year?"

"I will be once I send McGon—Professor McGonagall my book list," Harry said. "Would you like to join us? My treat."

"I still have some shopping to do," Andromeda said, "Teddy, you may stay if you like, but mind Harry!"

Harry snagged a chair from another table, and gave Teddy some money. "Go get your ice cream. We'll be out here."

Neville watched Teddy vanish into the shop. "I don't think I ever met Tonks properly," he said. "But he looks a lot like Lupin, doesn't he?"

"Yeah." Harry sighed. He'd been thinking about Lupin a lot—he had always had good advice, and Harry wished he could talk to him about being a teacher. It was both comforting and daunting to know that he was taking up the same position Lupin had held at Hogwarts. "Well, he does most of the time, anyway."

"He's gotten in trouble a few times for disguising himself as other students to play pranks," Neville said.

"Yeah, I heard."

"Who needs polyjuice potion when you're a metamorphmagus?" Neville said, grinning as he scraped up the last of the hot fudge from his bowl.

"That was one time," Harry began, but stopped when Teddy rejoined them, bearing an astonishingly large banana split.

When September 1st came around, Harry went to Hogwarts by floo network, appearing in the Defense Against the Dark Arts teacher's office—his office, now. He'd visited a few times over the summer to bring books and things, but he hadn't stayed long, and it still didn't really feel like his. It didn't feel like anyone's; the previous teacher had taken all his things away, of course, so the walls and shelves were bare, and all that was on the desk was a neat stack of Harry's plans for the first few weeks, and a note from McGonagall reminding him when to report to the Great Hall for the welcoming feast. That wasn't for hours; Harry had arrived early enough that the sun still slanted unfamiliarly through the windows, and he had the whole day to start setting up his office the way he wanted it.

In fact it took several weeks before it started to feel like his office. But in that time he surprised himself by settling into the rhythm of teaching, getting to know his students and learning what worked and what didn't. It was like the DA, as Neville had said, but it was also different—he had to make sure the fifth and seventh years were prepared for their O.W.L.s and N.E.W.T.s, and for the first couple of days he had to deal with wide eyes and awestruck reluctance to actually participate in class. But by the end of the month he was enjoying debates on spell theory with seventh year Ravenclaws, turning lessons into games for the first and second years (the second year Hufflepuffs had warmed up to him immediately, thanks to Teddy), and found himself offering chocolate to those O.W.L. students who were already starting to panic over the workload that year.

Then October rolled around, and Halloween approached. "Is it true you fought a troll your first year?" asked one of his second years, a Gryffindor named Estella. "Teddy Lupin said you did in Herbology yesterday, and Professor Longbottom wouldn't say if it was true."

Harry looked up from trying to find the list of jinxes he'd wanted to start going over. Every face in the room, Gryffindor and Slytherin, looked at him expectantly. "I did," he said after a moment. "Well, I didn't do it alone. And I wouldn't recommend any of you try it either," he added, trying to look stern. That only had them looking intrigued, however, so he sighed, setting the list aside. "All right, it was like this…"

Word spread quickly after that class, and Harry found himself relating the story to everyone, even his seventh years. It wasn't a very productive day, as far as learning learning jinxes went, but Professor Flitwick told him at the Halloween feast later that week that all of his students were much more enthusiastic than usual about the levitation charm.

Neville nudged him with an elbow. "Better than chasing down dark wizards, isn't it?" he asked.

Harry watched Teddy scrunch up his face to turn his hair from blue to bright bubblegum pink; he thought about his own children, and getting to see them get Sorted in a few years. "Yeah," he said. "Yeah, it is."