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.
In short, how can I tell these girls, politely, that they should think twice about showing up half naked to meetings with faculty members?
I can think of 4 situations:
If they are violating a university dress code, you should politely remind them of the policy.
If they are not in violation of a university policy, but their appearance makes you feel sexually harassed, you should follow whatever procedure the university has in place. If the dress code allows for clothing that makes you feel harassed, I would follow the procedure to the letter and not say anything directly to the students. If there is no dress code, you can politely mention that their appearance makes you feel uncomfortable (or you can follow the procedure).
If they are not in violation of a university policy and you do not feel sexually harassed, saying anything is giving them unsolicited advice. While I think it is not out of place for faculty members to give students unsolicited advice, you should do it politely and in a non-judgmental manner.
Maybe something along the lines of:
When meeting with someone in a professional setting business causal dress is often preferable, even when not formally required.
Finally, you may want to document the issue with someone in your department. While an extreme case, I had a student who would regularly unbutton her blouse prior to entering my office and button it upon leaving. She would do this immediately outside my door and in my view. This probably qualified as sexual harassment, but I did not care to follow up. I did, however, tell my head of school and director of teaching (as well as making sure my door was always open) so that they were aware of the issue in case she ever raised a complaint.
My suggestion is to do nothing and say nothing this term in order to avoid the risks others have pointed out, especially since you say you neither feel harassed nor uncomfortable.
However, next term, I would suggest doing what I had a professor do: add a "Professionalism" section to your syllabus and first day procedural talk. You still have to be careful in some of the ways others have pointed out: make sure you frame everything around professionalism and preparing them for the workplace, use subdued language that applies to unprofessional attire for both genders and if you cite examples, give one for men and women. Also, and this is key, include other unrelated examples of things they need to know for how to behave, like how to professionally email a professor (that's what my professor went on about. boring to me, but I'm sure some people in the class needed it). Don't forget to add something about how conforming to (for appeasing rhetorical purposes add 'arbitrary') rules about professional attire may be just a hoop to jump through, but that it is a socially meaningful way to communicate seriousness and respect. In general, just be careful of the tone. Preface and end it with something like 'most of you probably don't need this, but some students don't get the preparation they need in highschool for how to behave in the real world, yada yada yada...'
The benefits of this plan are many
- It gives you the opportunity to pass on actually helpful information to your students that they are expected to just know in the business environment
- It protects you from being misunderstood or maliciously attacked
- It accomplishes your goal or,
- Makes it so that if you do say something, it is less reasonable of them to claim you are overstepping or being oppressive since you made your expectations clear beforehand
- Gives you the opportunity to set expectations that will help the term run smoother and more enjoyably
Well, that's my two cents. Hope it helps.
Edit concerning addressing the issue this term (warning, somewhat abstract)
As someone in the comments pointed out, you could send out a mass email addressing the entire class. My concern with this gets at what I think the key distinction is in professionally responding to this scenario, and also provides an opportunity to respond to some of the critiques of the question itself leveled in other responses.
As many have pointed out there is a difference between professionalism and prudery. This distinction is not the same thing as whether or not student behavior (of any kind) bothers you. These are different aspects of the issue. Dress is a socially embedded method of communication, so it is unhelpful to say you merely ought to keep your opinions to yourself. Dress codes simplify dynamic, but are never exhaustively effectual. Taken together this means that professionally responding to scenarios concerning the professionalism of student behavior must take into account institutionalized norms (like dress codes), context (like your relation to students as well as timing), as well as personal judgements.
That last one is tricky though and that is where the fine line is. First of all, it is inescapable because we are socially embedded creatures. To say it is not inescapable by saying it is all merely opinion or that there is an absolutely objective response both lead to an arbitrariness as well as an abstract conception of what it means to be human which denies the real character of being a social creature both affected by and affecting norms. It is also problematic (ie, the risk of being a prude). This is all too philosophical though, so let me be concrete: it comes to not being disingenuous by simply imposing what you would like on your students, but making it about being helpful to them. Even if deep down you are disingenuous, it would be unprofessional to act that way, so you have to include various other pieces of advice on how to professionally behave as a student in a sufficiently non-confrontational, non-reactionary, and in an honest enough manner to actually help your students rather than to simply make them conform.
This means ultimately making it about the students and not oneself (even if deep down you're selfish about it). That means not being reactionary or making students feel singled out. That means you can't do it this term. It would be too reactionary causing those students to be self conscious about their looks, and would ultimately make it about you, which is morally condemnable, unhelpful to the students (I won't argue this point, but trust me), and (importantly seeing as this was asked within the context of academia), unprofessional. No pretensions about being objective would prevent this precisely because of its socially embedded nature.
As such, I repeat, do nothing this term, include a professionalism section next term, and try your best to use it to actually help your students. At least in my local, the highschools under-prepare their students, and you really may be the only person who ever explains to these students you are supposed to be respectful in an email, or turn off your phone before you go into a meeting with one's superior, and yes, wear sexually neutral clothing in a professional environment.