This is flawed.
Everybody knows all the hot girls get it by 16 anyways. Upperclassmen are always quick on the draw.
You become a legal adult at midnight the day of your birthday, it doesn't matter when you were born.
Also, these cases are decided on a case by case basis so theres definitly a gray area.
And statutory rape occurs when penetration happens.
So yes it would be considered statutory rape if penetration occured while she was 17 and you were older (general rule is its okay within 4 years but then again even that is case by case) even if she turned 18 during the act.
On a side note, if its consensual than your pretty much good assuming your within an understandable age-range.