Female students coming to office hours in overly revealing clothing
Does your institution have a student dress code?
Does your location have public indecency laws?

If their clothing violates either, refuse meetings until the problem is corrected.

Otherwise, ignore their appearance and carry on as usual.



Their sense of appropriate dress is clearly different from yours, but like political or religious opinions, such senses are often personal and cultural.  For all you know, a bikini is her preferred set of comfortable clothing.  Without a clear agreed-upon set of rules to defer to, such opinions are unproductive to dispute.  Let it be.