Ref
Their relationship isn't any stranger than your typical student-teacher dynamic.
Henry's thoughts on Sebastian
"He's a good student. He looks like he gets distracted during class, but he consistently does pretty good in tests."
Sebastian's thoughts on Henry
"He's just another one of the teachers here. I don't really get why so many of the girls are into him... girls are scary, man..."
Veronica's entire friend group has a crush on Henry. They pretty quickly learned that he isn't really opposed to doing... inappropriate things with them if they offer themselves willingly.
So they've been doing that. (In secret)
Henry's thoughts on Veronica
"She's... uhm... a very... interesting student. *nervous sweating*"
Veronica's thoughts on Henry
"O.M.G. He's so handsome and nice~ His fur is so shiny and soft too... I love him ♡"