I tend to agree with what Alias_Day was saying.
In almost all cases if a high school teacher was to date a student it would be illegal (regardless of the student/teach code of ethics). A teacher (or anyone for that matter) who would likely be at least 22 can't date someone under the age of 18 and do anything physical with them, by law...parents could give consent though. I've been in situations where teachers were having relationships with students and not only is it creepy, but not really fair. I understand that crushes will occur, those are natural...but by becoming romantically involved with one of your students you tend to give them advantages others don't.
An assistant band director at my high school (we had lots of issues with band directors...but had one of the best bands in the state, so I guess they were doing something right) got involved with a girl who was my age (this is while we were Sophomores). Not only did he act completely unprofessional around her, but she ended up getting "private lessons" from him when others couldn't, and ultimately got first chair, when she probably didn't deserve it. Word of the relationship got to the administration and he ended up being fired...so he only hurt himself.
While I personally still think it's a bit creepy, why not wait until after they've graduated and have become legal? My junior english teacher did that. He would flirt with female students all the time and would give them extra help (which bugged the heck out of me), but nothing relationship wise occurred. He is now dating a former student who is a couple years older than me. While he's double her age and was her former teacher, it's now legal...and however creepy it may seem, they do seem to really like each other.
If people want to date former teachers or former students, that's fine...whatever floats their boat. But I don't think any of that belongs at school. If the feelings they have for each other are that strong, then they'll still be there once the student graduates.