Not only was she in a position of authority, she could have ruined their lives. She was their English teacher. A failing grade from her could have ruined college chances or scholarships.
And no, the kids were not ******. Not even coerced, I am sure. They did it willingly. And they were 18.
However, she knew the law when she signed up to be a teacher. She is an adult with full command of her faculties. This requires obeying the law in a responsible way. You can disagree with the law all you want, but until it is changed, you still have to obey it.
The law is in place so teachers can't use the "well the girl/boy was over 18 when we did it. Prove I coerced her/him." defense. Yes, the absolute interpretation of the law can really be a bummer to some innocent fun. But the law is the law and she was fully aware she was risking her job, the kids' future, jail time, being banned from teaching for life, and having to register as a sex offender for the rest of her life, when she took the job.
Must have been some good sex to risk all that.
Oh. and she was not just risking 20 years. She was risking 20 years on EACH of 16 counts against her.