Can somebody simplify this for me?

Anyone who is 17 or over can legally have with anyone else 17 or over, except that a school employee cannot have with a student, regardless of the student's age. If the minor is under 17, the older person can ert as a defense that he/she is not more than 3 years older, of the opposite , and did not use duress, force, or a threat. Since this is an affirmative defense, it must be proved by the defendant beyond a reasonable doubt.