Obviously, you are referring to high schools on this post but I just looked out of curiosity- the only ODAC school with grass is Hamden Sydney. College stadiums are roughly 50-50 at the bigger D1 colleges and most all smaller schools have turf. I believe the versatility of the artificial field makes it much more advantageous to all outdoor sports teams. The upfront cost is fairly high, but the general upkeep is minimal and you will continue to see more and more schools go to the artificial route.