As clarification as to why I, personally, object to this story...
I get it. Forbidden love storylines have a certain appeal to them. However, there is an issue with the message you're trying to convey.
If a 20 year old were to be in a relationship with say, a 10 year old, that would be 10 years of difference between them. That 10 year old is a child. The 20 year old is an adult. That is considered pedophilia and that is inethical.
Even if it's a highschool student and his or her teacher, it's still... not right. A teacher has a moral and legal responsibility to act ethically and properly around their students. Most highschool teachers are at the very least in their thirties, and highschool students are usually anywhere from 14 to 18 or even 19. So when you have a teacher-student romance, that is at least a ten year gap between them, and as the student is still legally a minor until they graduate highschool, that's still pedophilia.
Edit: I'm saying this because I have dealt with actual pedophiles before. Because it's an actual problem in the world.