this isn't so much a headcanon more like a scenario that i believe is completely and totally canon
so kind of like a headcanon

~ The Bakusquad, sitting together at lunch ~

Bakugou: "When I was 7 I had a crush on a boy with black hair in my class and didn't know how to deal with it, so I wrote him a letter that just said, "Get out of my school."

Kirishima: *shOOTS UP FROM HIS SEAT* "THAT WAS YOU?!"