There's no single answer to the average US high school tuition because public high schools are tuition-free. The cost is covered by state and local taxes. The question only applies to private high schools, and even then the average cost varies wildly depending on the type of school (religious, secular, boarding, day), location, and other factors.
Therefore, you won't find a single, universally agreed-upon "average" tuition for US high schools. You'll find a range of figures from various sources, and those figures will represent averages for specific subsets of private high schools.