Whatever you do, I would NOT take anything out of a college fund, nor would I stop contributing to a college fund. I also would absolutely avoid taking on any debt for high school.
From my experience attending a big name university for undergrad, most of the students there were from public high schools. Some were from rather bad public high schools. The issue was not where they went to high school. It was that they had excelled in high school wherever they were.
If you can afford prep school that is great. However, you really do not need to attend one to do well in life.