Wudge, with all due respect, it's you who's missed the point. The age is totally irrelevant. The point is that the teacher has unilateral power over the student, and that power is conferred upon her by, and backed by, the authority of the state under penalty of imprisonment. The student is not in a position to do what he pleases in the classroom no matter how old he is. Even if the student were 50 and the teacher were 19, the student would still be subject to laws regarding behavior in the classroom that are enforced by the teacher and backed by the school district police; his grades are still under the unilateral control of the teacher with no right to appeal those grades; and whether he graduates or not (and therefore gets to go to college or get any job that requires a degree) is under the control of the teacher. Students' grades and diplomas should be based on how well all of the students do on tests -- not on which of the students agreed to have sex with the teacher and which did not.
It is the same reason why prison guards cannot have sex with prisoners; judges cannot have sex with defendants who appear before them; state child welfare officers cannot have sex with the mothers of the children they supervise; parole officers cannot have sex with parolees they supervise. It's not the age that matters; no matter how old any of these people are, the fact is that in all of these scenarios, one party has enormous power over the other, and the other party is not free to simply walk away and avoid that person in the future because the government forces him or her to continue to interact with that person under penalty of imprisonment.
The point is that decisions about grades, or prison time, or freedom, or whether your child is taken away from you, or whether you get a diploma, or whether you're convicted of a crime, or whether you pass your driving test, or whether you get audited by the IRS, or whether you can get a marriage license -- or any other benefit or punishment that government can confer or deny on you -- should be made on the merits of the decision, not on whether someone consented or refused to have sex with you.