Hi,
With the rapid change in attire attitude, there are times when a very thin line exists between formal and casual dress. Also, there is Friday dressing, so I suggest that you create a policy that mentions generic dress like round neck T-shirts, cargos, denims, skirts that are too fancy, etc., are casual. 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 as it is not distracting and vulgar. It's surprising to dictate terms of dressing to mature individuals like school kids. I hope we need to focus on creating a 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 :)