I don’t think picketing is a good idea. It will just create more problems and sinas chinum.
Let’s get to the source of the problem.
Girls in school need to be taught the halachas and motivated (I have given an idea for that earlier in the thread) to follow halachas.
Mothers need to be role models by following halacha and dressing well. But even if a mother does not care too much about fashion and doesn’t care to be fashionable (to each his own) a mother must convey the importance of being a tznua to her daughter.
Only if daughters respect their mothers will they take their mothers seriously. Having a kesher with a daughter and being sincere is of paramount importance in a mother/daughter relationship. (Or parent/child retionship for that matter, as well.)
Also if mothers of boys in shidduchim feel that looks in a girl is of primary importance, girls take notice and feel that they need to flaunt themselves. So mothers of boys in shidduchim if you have daughters and you want your daughters to be b’tznuas it’s important that you show what true values are.
All above is very important, but the most effective way to combat pritzus is the school laying down the laws of tznius standards the students need to meet to be accepted in their school and those standards need to be met whether the student is in school, at home or anywhere else.
Of course the school dress code need not be the same as the tznius standards which may mean that different clothing may be accpeted to wear at home and not at school. Clothing kids wear at home can be more casual than say uniforms, but the tznius standards for dress at home have to be set according to halacha.