Well, at my school we have a uniform *sad face* and it is only a boys school *even sadder face*.
But back on topic, just because some dudes enjoy staring at a girl's ass in yoga pants the whole period, doesn't mean that the pants are wrong. It means that there is something wrong with the dudes.
Anyway, personally I think that the banning is for a deeper reason. Maybe there was this dude who grabbed a girl's ass in school (But he must be stupid or something, it would have been much better in a dark alley *hepraderpaderp*) and when he was confronted about it, blamed it on the pants.