katsuki is you maths teacher but also your secret boyfriend. No one else knows about you two dating since you were his student and he was his teacher and it was illegal. Even though he was your teacher you were close in age, you were 18 and he was 20. Your sat at the back of the class, all the students were looking towards the front as he was teaching but you were staring directly at him and he kept glancing at you

“So if you look here, you can find the square route of ten”