He watched as they left, before following them quietly to make sure they got out.
"I'll escort the two of you out." He said, as he followed the two girls out of the building. The boy prefect paused for a bit, before then adding. "Oh yeah. Try to have your excuses ready before hand, so I don't actually see you taking out your phone from your pocket." Just to make sure they knew he didn't buy their excuse one bit.
Minato was forever sleepy, yes but that didn't mean he didn't notice the small details that showed up right in front of him. He wasn't that type of person to let small details go unnoticed. Such as the blond girl most likely was forced into this plot by her friend, since she was the one who looked like a deer in headlights when he caught the two of them in the room.
Also, since the girl-Yumiko? was often one of the day class girls outside by now, waiting for the dramatic exit of the vampires that waited inside. Small facts like that could easily be seen, even without his training.