My girls know what I expect and that clothes have to pass the momma-test. They can be flattering but not tight. It is amazing what is passed off for children's clothing.
What you said is a big part of the problem. Do the parents of those teens not realize that they are the parents? I mean, who runs their households, the kids?
Kudos to you for being an adult and a parent, rather than part of the kids' peer group.