I'm not 100% opposed to it, but it can get really sloppy really fast. There's a lot of potential for students to use the professor for a better grade or the student to feel obligated at risk of hurting their grade.
I mean I dont really mind when college teachers date their students. They both are adults. But show your affection in your free time please, these things dont belong to school. Teen and teacher : HELL NO.
Honestly if the student is of age, I don't see why not tbh. Though, unless if the teacher worked in that school, but the said student didn't take the class he's teaching, because there's definitely no way there wouldn't be a conflict of interest then