In High School education, Factorials are generally part of the curriculum, but they're one of those things you get one section on, it shows up on one test, then in common usage, you never see it again. In many schools, someone could have been out for a day, gotten two answers wrong on a test, and never have known it. Then in my school, unless you were heading on a math track, you'd hardly even touch Calculus to see it actually used anywhere.