"Little girl has a one-sided crush on older neighbor boy."
okay
"Boy says he'll marry girl if she grows up into his type of girl, so as to not ruin her first crush."
alright
"Girl grew up and now she's kind of over it but the boy is interested in her, and they're both old enough that it isn't too weird."
getting good...
"HE'S GONNA BE HER TEACHER AND IT'S GONNA BE AN ILLICIT STUDENT-TEACHER RELATIONSHIP!"
why
You can make a good romance without making it taboo, Japan, I know you can, I've seen you do it before.