Possibly these 15% are the ones who really care.
One of my professors told us on the first lecture: "I'm not holding anyone here. The course I'm teaching [information theory] will be practically important to few of you. I don't need anyone here who does not really care. Don't distract and slow down those who do care. Go now." About 50% of audience walked away.
I didn't. It was a good, fast, thick course. Those who did not attend used books and mostly ended up with B's.