"Why would I pretend to be something I'm not?" Isaac was intelligent enough to know he should have just shut his mouth, but knowing the right thing to do and doing it were vastly different things. He was already in trouble, so what was the harm in seeing it through, and maybe making Killian uncomfortable in the process.
"I thought your boy-wonder fixed the world so no one would have to do that anymore?" Being a pureblood and a prat, Isaac had no qualms bringing up blood heritage to prove a point. Not that he much cared about the fact that his mother and father were both magical, as far as he was concerned, his mum worked too much, and his absentee dad was a dick; that was all there was to know. Still, he brought it up anyway and dared Killian to respond with a quick raise of his eyebrow.
Isaac knew the write up was coming and that it would lead to detention or docked points, but he was in too deep to back down now. By some logic, the teenager had convinced himself that if he was going to be punished, he might as well do something worth getting into trouble for.