Based on a prompt I got on tumblr!
This is set a few decades in the future of AnE! Also, here "Kin" means related to a Demon King. This doesn't mean the Demon King himself necessarily but can also refer to demons associated with said Demon King.

Katsuki Bakugou, 15 years old, kin of Ilbis, Exorcist in training and utterly bored. When he had signed up for the advanced exorcism class instead of just the mandatory one, fully set on actually working in the True Cross Order, he had expected glorious battles - not boring lessons. All the hope he had put on the classes for those who were kin with demons had been shattered within a day too. School was boring. It was so much mindless studying and recitation and remembering- the kind of things Deku was interested in but never Katsuki.

As Miwa sensei lectured, one by one everyone fell asleep. Even Deku was distracting himself by growing some weird ass plants on his arm. When he was doing things like that, it was hard to imagine that somewhere blood of the crazy and weird as hell Earth King was running through the teenager's veins. Well, not that he was the only one, Satou and Mineta were related to the Earth King as well, but Deku was the only one who used his powers so weirdly. Perhaps he was the only one who could use his powers like that. Rumors had gone up that Deku was actually related to Amaimon, though Katsuki had yet to see prove.

Sighing he dropped his head on his desk. This class was so boring.

"Pssssht!"

Katsuki turned to his left. His seat neighbor, Egan- something he couldn't recall, waved at him while smiling the probably biggest smile Katsuki had ever seen.

What the fuck.

"You're bored too?" Egan asked.

For a moment Katsuki considered ignoring him, but finally decided against it.

"Yeah."

"Awesome! Well- not really, but yeah. Mako- Miwa-sensei is a real bore, right?"

Katsuki could only nod. "The only really interesting lessons are Aizawa and Toshinori's."

At that Egan nodded. "Yes! Aizawa has some awesome Tamer stuff going on. His Nebari summons are hella creepy though. Toshinori's Aria lessons are fun too! Do you have demon pharmaceutics? Yukio's classes can be boring but you should-"

"EGIN!" Miwa interrupted. "Don't interrupt class!"

"Yes, sir!" Egan- Egin shouted, waking everyone up who was asleep.

What a weirdo.
He was bearable though.

The next weeks continued in similar matter. Whenever Katsuki was about to dose off, Egin would start a conversation until the teacher called them out. Sometimes they only talked for a few minutes, on other days for the whole lesson. And somehow, whenever Katsuki wasn't bothered by Kaminari and Kirishima, Egin would hang out with him or worse: drag him to the rest of his classmates and have him socialize with them. Katsuki didn't even know how Egin managed it. One moment the blonde was alone, the next he was sitting in a circle with the rest of the demon kin teenagers.
At least he got some more information on Egin, who was weirdly blank. Yukio Egin, the pharmaceutics teacher, was apparently his older brother by ten years or so and Egin wanted to gain the Meister Knight and Aria. Egin was also shit at any recitations so the Aria Meister didn't make a lot of sense to Katsuki until he heard the other teenager sing a whole lot of Latin bible phrases and prayers and what not. He honestly hadn't expected that from a guy that sucked at remembering something as much as Egin.

The thing that interested Katsuki the most though, was who Egin was kin with. He had to be kin with some Demon King, otherwise he wouldn't be in the same class as Katsuki.
The summons Egin made weren't really helpful solving that riddle either. Usually one stuck to their own kin and maybe another one that fit. But Egin apparently was the prime example for "screw the rules" as he summoned demons of all kinds and classes, often earning disapproving glances from Aizawa. But about that Egin didn't seem to care too much. In general, even though he was in the top five, Egin seemed so give zero fucks about school. It was odd since he was such a straightforward and determined guy.

"Katsuki!" Egin called and waved at him. "Class is about to start!"

"I'm coming, idiot," Katsuki said and dragged himself towards the gym.

Inside Egin had already managed to get into a heated argument with Kaminari on how good Egin actually was at summoning. It was starting to look a bit like a small or, hell, maybe even big disaster as Egin ran over to the only blackboard in the gym, a piece of chalk in hand. He erased the previously instructions on how to properly evade a demon to make place.

"I'll show you then!" Egin declared and began to sketch a circle Katsuki wasn't even sure of to what element it belonged.

"Bane of ancient times, here my call, by the blood of my kin, come forth!"

Not that he'd ever admit it, but what happened next scared him just a bit. One moment Egin appeared happy and successful, the next he was plucked from the ground and in that beast's mouth. His classmates started screaming, terror slowly crawled inside the previously peaceful classroom. So this was actual combat.

No wonder they were still going to school.

Egin's demon was terrifying. About five meters tall with at least a hundred yellow snake like eyes. Its mouth made up at least half of its body in total, the rest being a weird chimera like mix. The knowledge he had acquired in school so far couldn't help him determine what this thing was. It radiated warmth, had spider legs, red scales, messy dark fur- it was a living nightmare. One that was about to kill Egin.
Whatever courage Katsuki could muster up, he poured into his next action. He made his way towards the beast, fire already biting at his palms. No matter what this thing was, surely it would hurt it if Katsuki burned his eyes?
He was nearly there when Asui hold him back.

"What the-"

"Your fire won't help," she informed him. "I can feel it. That thing is fire."

"How would you know?!" Katsuki shouted. "That thing will kill Egin!"

"I'm kin with Egyn," she told him, appearing rather unbothered by his temper. "And so far it hasn't done much. It looks like it's waiting for an attack."

The worst was that Asui had a point. Recklessly and mindlessly storming into battle, wasn't that something Egin had also told him to get under control. Katsuki turned to look at the demon again. It definitely was waiting for something as it made no move. It didn't even try to devour Egin.

"What are we supposed to do then?" He asked.

"Iida's run for a teacher and the rest," she pointed to the other students, „are working on a strategy. Just because you have the fastest reaction, doesn't mean the rest of us won't do anything."

"Fine," Katsuki bit out just the moment the demon suddenly screamed.

Everyone jumped with fright, expecting the beast to suddenly lose control and go ahead and attack them. For one moment, nobody reacted. It felt as if everyone even stopped breathing, completely frozen.

"That's enough."

From behind the students, at the gym's entrance, Egin's older brother stood. Instead of looking panicked or even just worried, he appeared annoyed.

"Nii-san, stop with the act. You've had your fun. Now stop before the class seriously hurts themselves."

"Spoilsport!" Egin's voice rang through the building.

Katsuki couldn't quite believe his eyes. The demon carefully dropped the completely uninjured Egin to the ground.

"You're always ruining my classes. And do I have to remind you that your exam nearly got me killed?" Egin huffed as if moments before he hadn't been dying.

"I didn't even come up with your Exwire exam! It's been decades! You can't still be holding that against me, Rin!"

"I can and I will. Now-"

"What the fuck is going on?!" Katsuki expressed what all the other teenagers were asking themselves. Why was Egin alive and well and complaining about his brother ruining his classes.

"I'd like to know that as well," Todoroki added while a bunch of their classmates nodded.

"This, my dear students, was your exam!" Egin exclaimed cheerily. "Allow me that I reintroduce myself. Okumura Rin, Honorable Knight and a bunch of other titles making me the glorified babysitter of my siblings, at your service. I'm your actual teacher, not Mako-kun. He was just my step in so I could observe whether you'd be worth my time."

"Liar," Egin- Okumura's older brother said. "You would have taken the class regardless of what impression they make on you as long as it gets you away from your paperwork."

"Shut it, Yukio. Not everyone can do paperwork for nearly a century and not go mad. You're weird like that. Anyway, Ashido, you have to be careful with your summons. I'd stick to weaker once for now in stressful situations. Todoroki, just because you have a bit more power than the rest doesn't mean you can-"

And Egin- Okumura latched into criticizing and/or praising how they all reacted and behaved just like a freaking teacher. Somehow Katsuki still couldn't fathom what the fuck was going on but apparently that was alright because neither could his classmates.

Basically (semi) immortal Okumura twins teaching Cram school and hiding their identities because have you tried going anywhere when everyone knows your name? Nah.