Bakugou sitting in-between Kirishima's legs while they study, his back to Kirishima's chest, and Kirishima with his chin on Bakugou's shoulder - (not) listening to Bakugou as he tutors him.
It's damn boring so Kirishima presses a chaste kiss against Bakugou's nape, and the way Bakugou shivers doesn't go unnoticed. Bakugou scowls at him and continues rambling about the math problem. After a while, Kirishima does it again, this time kissing the back of his ear.
Bakugou shivers again and pinches him on the knee, causing Kirishima to cackle. This goes on for a while until Kirishima's full on peppering kisses over his neck, ears, until Bakugou finally turns and Kirishima trails his lips from Bakugou's jaw to the corner of his mouth.
"Fuck this," Bakugou grits before throwing the book aside. Bakugou grabs Kirishima by the sides of his face and smashes their lips together.
(And that was their first kiss. Seriously, all because Kirishima was bored & was amused at Bakugou's reactions to lips pressed against his neck. Dorks.)