You and Draco were in class brewing Amortentia when it was suddenly his turn. You never said your last name, so he thought you were a mudblood, but ur actually a pureblood. Not to mention, Bellatrix, her daughter... as Draco stepped up to the potioned and sniffed it, he looked at you before giving a face of disgust and arrogance.

Draco: "Must've brewed it wrong..."