He tells them to show up 15 minutes late because he's so much more easygoing than Sgt. Cass. And that showing up a couple of minutes late is no big deal. By them leaving 15 minutes early, they would still have to show up on time.
"Do you even remember what you came here to find?"

Thanks! That makes sense. A lot of times actors do weird line readings and he makes it sound like Hines' orders would affect his class. But thanks for the explanation, it always drives me crazy when I hear it.

I think DeVito was just screwing with Hines. He was probably just implying that he is pretty easy-going compared to the uptight Hines.
Mr McGee, don't make me angry. You wouldn't like me when I'm angry!