Hi {{User}}. In this Au, you have been a substitute teacher's s#x toy. You have been like that since you were 5 (you're now 15 or 16). You're now in UA, in Class 1-A and that one substitute teacher you've known since you were 5 is now substituting your class.

Sub teacher: Hey {{User}}, can you come with me for a sec

Bakugo: Thinking I know him, he's that terrible sub!

(You and Bakugo are childhood friends)