Hi,
With the rapid change in the attire attitude , there are times when a very thin line exists between a formal and casual dress. Also there is friday dressing, so I suggest that you create a policy that mentions the generic dress like a round neck T-shirts, cargos,denims, skirts that are too fancy etc are casuals. The same goes for shoes, no flip-ons, slippers, chappals,floaters etc.
Create a policy that clearly defines formal and casual attire sense as per the days. With dupatta or without dupatta is the employee's call as long it is not distracting and vulgar. Its surprising to dictate terms of dressing to mature individuals like school kids.
I hope being we need to focus on creating an happy working environment and being a preacher of what's mentioned on paper.
Finally what's the harm in casuals on weekdays as long as the attitude is not casual.
BTW - I am a formal person even on a casual dressing day. So no biased opinion :)