They dont have to. By excluding families (from schools for example) that do not dress this way and labeling them “untznius” is more than enough.
That is not called pushing chumros on other people. If a community wants to accept on its self a certain standard of tznius it has that right. No one is being held at gunpoint to send their children to these Yeshivos/Beis Yakovs. If you are not comfortable dressing your children according to the tznius requirements of a certain school, send your children elsewhere.
Rabbeinu Tam tefillin is also for ballei madrega, would you say someone who doesnt wear these tefillin are not following halacha?
Do you think that a community that accepts on itself a certain level of tznius has to accept on itself all practices of ba’alei madreigah.
Tosafos in tznius is something that all chareidi communities accepted on themselves. Rabbeinu Tam teffilin is not.
I have not seen anyone label dressing a certain way “madreigadik”, just tsniusdik.
That comment was directed at an earlier post that appears to have been deleted.