Headcanon: Class 1-A comes up with a sign-language only they can understand.
It’s pure fun at first, really. It’s not even planned to be a sign-language. Signs and gestures come up in class when Mina imitates Iida wild hand gestures one day and Sero immediately knows what she’s trying to say even when she didn’t say it verbally.
The class howls with laughter at first – even Iida smiles at his silly classmates, knowing that they’re not trying to make fun of him, just having fun.
From then on, it kind of continues fluently. For days, Class 1-A imitates Iida’s gesturing, finding it funny that simple gestures can mean so much and be understood so well. Over time, there come gestures and signs up that Iida didn’t use. It’s a mixture of gestures everyone knows – flipping someone off is something that originates from Bakugou, and the thumps-up to say “okay” comes from both All Might and Izuku - and others that just get invented along the way.
It’s Izuku who first notices that this could be an actual sign-language. He notices it while observing his classmates, and he starts thinking about using it for training, since they all train together. When he proposes that to his friends, their all for it.
The more enthusiastic ones like Mina, Kirishima and Tooru are out of their minds with giddiness, instantly wanting to add new gestures and “Let us help, too, Mido-chan!”. Tooru likes the idea that, since her facial expressions are not visible to her friends, she at least could express herself non-verbally by gestures.
The more quieter and rational thinking peers like Todoroki, Iida, Momo and even Bakugou have to agree that this could be useful at some point. It would give them an option to communicate fast and over some distances during battles, and on top of that, the opponents couldn’t understand them.
From there on out, Class 1-A spends most of their free time to design their own sign-language. They come up with signs that mean single words, and some that carry a whole sentence as a message. Izuku is the one who writes everything down in his notes, but all twenty students help to add to their new language.
With all their combined effort, it doesn’t take long for them to have a working, unique language. And before you know it, the kids are communicating in sign-language, not only during fights, but also at the dorms.
Toshinori and Aizawa are quite confused when they first witness the students signing something to each other, hands moving at high-speed, before they nod at each other as if they had just agreed on something. Aizawa wants to observe further, but Toshinori is faster and just straight out asks the children what is going on here.
Of course, his students are delighted to include their teachers in this, and they explain in detail how they came up with their own language and how it works. Toshinori is excited about it, immediately asking if he could learn it, too. Aizawa grumbles a bit about his colleague being a big child, but he confesses that he is interested in learning, too.
It doesn’t take long until class 1-A plus teachers are able to communicate non-verbally and easily with each other. Their opponents doesn’t stand a chance in any following teamfights anymore, since the teamwork between the members in this class is almost too perfect.
But the sign-language is also quite nice to use in their free time, for example, let’s say, during an exam (which is not observed by Aizawa or Toshinori):
Momo? First question?
Number 2 is the right one
Stop fucking cheating, guys.
It’s teamwork, buddy.
Kacchan, you just helped Kirishima-kun?
That’s not cheating, that’s tutoring. Hair-for-Brain sucks at studying.
Like you too, man. Like you too.
Sensei is coming over, stop signing!