I spent Pessach in Petach Tikva. One of the things I notices were a few of the younger (15) kids in shul were wearing cloth bracelets that said "שמירת העיניים היא השמירה שלי" literally - Keeping my eyes modest is my protection. When I asked one of the kids, he told me that a rabbi in his yeshivah was making them wear the armband, after giving a Tzniut shiur.
I was both amused and somewhat repelled. At age 15, I'm fairly sure these kids really do need a good Tzniut shiur. On the other hand - wearing an armband would seem to be a constant reminder..which may actually backfire. At some point making such a big deal of the issue, serves only to bring it to the forefront of their minds.
I have yet to make up my mind, what I think of these armbands, though I'm sure my own children will not be wearing them at age 15.