It should be illegal, especially because she's in a position of power over the students.
How would you feel if your daughter had sex with a teacher her senior year so that she would graduate? At least, that's what her teacher told her.
Id want him fired, but i certainly wouldnt say it deserved jail time. Especially not a felony. She isnt a threat to society because she had sex with an 18 year old. We dont need to be putting her in jail.
And even if you should have a criminal situation because of the power she has, the power should have to be used. Completely consensual sex with no coercion? Not so much. And what about bosses and their employees? Should a boss go to jail anytime he or she has sex with an employee? He's in a position of power over another adult. No difference.