I was thinking more of the scar being an old injury that happened to be in the right place but it being the wing opening/emerging sounds cool
If we go with that then I have an idea that just popped into my brain
Iruma had been sore for while now but with all the pressure place onto the abnormal classes about grades, he didn't dwell on it much
He'd had a long talk with Ali after the surprise of the ring becoming sentient passed and the two of them were almost positive the pointed ears and fangs were the only changes Iruma would go through
Iruma though it came from a combination of living in the Netherworld and having Sullivan's magic into the ring
Ali didn't deny those assumption but he also didn't really confirmed them, he hadn't been 'alive' long enough to be knowledgeable about this type of things but something about the situation felt wrong with him. Still, Iruma was fine with the changes, it made him blend in more easily, and if sometimes he acted a little more like a demon that should be normal for a human, well, nobody seemed to have noticed (except Ali, he'd noticed, but he tried to ignore the possible implications)
So Iruma had put the pains at the very back of his mind, like he'd done many times in the past, it wasn't anything new. He just secretly hoped they were growing pains and that he would hit that long awaited growth spurt he'd heard teenagers were supposed to go through at one point.
That why, when Balam-sensei clapped him on the back, he really didn't expect the pain to be so great he'd actually cry out.
Balam-sensei panicked immediately and didn't even let Az or Sarah reach him before Iruma was whisked away to the teacher's room with a cry of 'please tell me I didn't permanently injured a student's wings' that Iruma was perplexed about for a minute
And then the full meaning of the words came to him just as Kalego-sensei approached to lift his shirt and he could do nothing stop him
He opened his eyes after a long minute of deafening silence, and when he looked above his shoulder the sensei's face were a varied mix of pained, sad and understanding
They let him go with an apology and asked Balam-sensei to stay behind for a little talk
Iruma had absolutely no idea what just happened and why they didn't just freak out at his lack of wings, he was so confused he didn't even remember the two scars running from the top of his shoulder blades to the middle of his back until he was ready for bed
Meanwhile you have the teachers quietly talking between themselves about how it had been at least a century since they'd seen a demon with that particular desease that made a demon's wings grow really late in life
The disease wasn't fatal by any means, but in a world in which everyone could fly almost since birth, the kids that contracted it were more often than not killed right away, or just abandoned into the wilderness
They figured the second situation was what actually happened to Iruma and he was then adopted by Sullivan, hence why they didn't look alike physically
They agreed to never talk to anyone else about it, since it was obvious Iruma would soon get his wings, and Kalego was nicer after finding out, feeling guilty about literally pushing him to what could have been his death during the flying race at the start of the year
..... And I got carried away again, thanks for that wonderful food for thoughts tho, I hope this idea is okay with you! :D
You can imagine Iruma will have a long talk with Ali about what happened, and then he'll ask Balam-sensei about the teachers reactions and learn about the desease, he'll then be conflicted between telling Balam he's human or keeping that secret and rolling with the desease idea so he can freely ask more about it
