Well, prison time might be over the top but Imo there are good reason for keeping relations between teachers and students on a professional level.
"Might" be over the top? Uh... yeah.. it is. Just fire a teacher that has sex with a student, at most take her teaching license for X amount of time, maybe forever. Prison time is
definitely over the top. In the eyes of the law, she raped a minor. The case is a "bit" different from hairy forty year old men who rape ten year old girls.
Also, in America, at the age of seventeen you're entirely capable, in the eyes of the law and society, of firing assault rifles, throwing grenades, being tortured and going to war to kill and die for your country. But sex? At sixteen? Get out of here, rapist!