what impact is this likely to have on the girls' self-image and view of their own importance in the world?

If a girl isn't a cheerleader, some become "Pepettes" who are assigned to serve one football player. Although accepted, it's pretty demeaning and demoralizing. It teaches girls that they are merely worthy to be servants of men, or boys, deemed to hold power. It is misogynistic and damaging to girls.