I was recently told of a policy in an Israeli religious high-school, where the male teachers are not allowed to ever remark to a girl student about a lack of modesty - clothing or otherwise. I'm not sure what the rationale behind this excellent policy is, but I assume it is a combination of all three:
a. The very act of commenting on Tznius by a male to a female is somewhat not Tzniusdic.
b. Its none of the men's business.
c. In matters of clothing etc, girls are more likely to respond to female teachers than males.
The conversation then progressed to asking whether this policy should be adopted in all of the Dati-Leumi public - let the argument be among women and not men criticizing women. I'm not sure I would go quite as far as that, men clearly are part of the Tznius story, however the spirit of the idea - and specifically that commenting on another person's Tznius is by itself a non Tzniudic act - should certainly be more prominent message. .