There are some pertinent facts to this case:
1) she was a teacher's ASSISTANT in a special ed class. The 18 yr old was not in the class
2) the 18 yr old consented to the sex
3) nothing happened at the school.
North Carolina law forbids teachers from engaging in sexual activity with students at their schools.
So charges could be dropped, as this could be invasion of privacy. Read this:
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Why the charges might not hold up.
And no-so-pertinent: she is engaged to another (fulltime) teacher @ the same school.