In most states, Age of Consent is 16, but there are exceptions... including those who have authority over the teen, most especially teachers, police, counselors, etc. I'm betting that's where she sent wrong.
Emily Lofing sentenced 90 days: Nebraska teacher gets away having sex with 16 year old student
But it still doesn't really explain the relevant Nebraska law.
The lenient sentence comes due to Nebraska state law allowing any 16 year old to consent to sex, even with a teacher. Nevertheless teachers because of their role as educators are strictly forbidden to fraternize with students, never mind sexually prey on them, except that is in the state of Nebraska.