Average? Probably 55 or so. There are so many small schools that don't have a good track team where a 55 will be the top guy, that it outweighs the good teams where you practically need to break 50 to get on. My high school was never a sprints powerhouse, but I can't remember a year where we weren't one of the best teams in the section, which included probably ~100 schools. Making our 4x4 usually took 52 or 53, and if we were in the top 5 every year then that's nearly 100 teams that were worse than us, so 52 to make it is definitely not the average.