Is everyone really that immature that the attractive posterior of the girl in front of them is such a distraction that it causes them to fail? I'm calling bullshit. No matter how nice a girl looks in some yoga pants, it takes a particularly blank individual to be able to stare for an entire class period and end up failing the class.
I have a different theory: Most girls who are incredibly attractive (not all) are not so intelligent. Girls who are incredibly attractive tend to wear yoga pants. This would mean that the boys that share classes with them are not that intelligent either. Chances are, these dumb boys would fail the class anyway. Instead of hiring better teachers and making better attempts to teach the students, the teachers don't want to take blame and would rather point fingers at the girls in yoga pants. So they did.
'Nuff said.