I'll be in the minority here. Freedom of expression is okay, as long as essential body parts are covered. These are kids, still developing a sense of style. They should wear what they choose, with the level of parental supervision they have.
The best way to end slutty-looking choices would be to clean up the style choices of so many in the entertainment industry, who are presented to these kids as fashion icons.
My freedom of expression was intercepted by a nun with a needle and thread who took two stitches in my prom dress neckline. Did not bother me because we all expected the inspection pre prom banquet and dance. That gown would still be stylish today and I would probably take the stitches myself. My girls pushed the expected boundaries for apparel every chance they got but I had the final word, at least until they were out of sight.