Blaine is a sub at Dalton. Sebastian is one of the few doms (possibly a switch?). They are in a short-term contract. Kurt arrives at the school and handles Blaine better than Sebastian. It's Klaine endgame. It's not "Disguise our Bondage" by alura_embry, although I adore that one, too.
ETA: The story is "Braided" by MissBeizy and the link is in the comments.