I know that the OP is in NC, but for the 1/8 of U.S. folks who are in California, here are the rules:
Bylaw 204. B. Eight Consecutive Semester Rule. A student who first enters the 9th grade of any school following the student's completion of the 8th grade in any school may be eligible for athletic competition during a maximum period of time that is not to exceed eight consecutive semesters following the initial enrollment in the 9th grade of any school, and eligibility must be used during the student's first eight consecutive semesters of enrollment at that school or any other school.
So once you start high school, you can't reclass backwards to gain a year of athletic eligibility. You can reclass forward (skip a grade).
As Kevin A alluded to, the way this restriction is gotten around, is to reclass in 7th or 8th grade. The age limitations in California are:
Bylaw 203. Age Requirement. A student, whose 19th birthday is attained prior to June 15 , shall not participate or practice on any team in the following school year. A student, whose 19th birthday is on or before June 14, is ineligible.
Lately there has been a phenomenon (particularly in basketball, in which there a few prominent recent examples) of "double holdbacks," in which kids with birthdays that fall from June 15 - Aug. 31 (and who would normally graduate high school at age 17, turning 18 in the summer) are held back not one year, but two. So they play their senior year at age 19, and turn 20 over the summer right after high school graduation. But all of the holdbacks must happen before they start 9th grade.