He's there. I'm going to play it as the Headmaster and the Lord have an understanding with each other.
Basically, a Knight administrates the Death School final practical exam every year. There are a lot of Knights, and they work for the Lord, and the Lord is the death of all thing. When the universe ends, he's the one that turns off the lights, puts up the chairs, and locks the door on his way out. It's a big deal.
If that's all right with Marv, of course.
Hey son, wanna' learn how ta' make witch balls?