You can only make qualified withdrawals from a 529 for post-secondary education expenses. Paying for high school would be a NQW, and any earnings would be subject to tax and penalty.
You might be thinking of using the 529 to take advantage of the unlimited SC state tax deduction. Wth an immediate withdrawal, there should be no earnings to be taxed or penalized and you can effectively pay the private school education with pretax (statewise) dollars. You need to check if SC recaptures any previously claimed deductions for NQWs - many states do. That information could be in the state's disclosure documents or should be known by a tax professional in your state. Even if SC does not recapture deductions for NQWs, this really isn't how 529 plans are intended to be used.
[This message has been edited by tjb_nc (edited March 26, 2003).]