...why is the younger party consenting? Do they just really like the teacher and are having fun? Then who cares? Impress me? Also, who cares? It's not like people care when kids try to impress older people in other ways. Avoid punishment? That sounds like blackmail, basically not real consent and thus punishable.

That's fine if those aren't reasons you want to get sex or relationships. I happen to agree. I prefer a relationship for mutual pleasure, because we like/love each other, and yes to satiate curiosity. I wouldn't be bothered to create a scenario that incites curiosity, but I don't do it for obvious legal reasons.