Think BIG:
Being that you are in girl’s chinuch (you said you work in a Bais Yaakov, no?), I’ll concede the expertise on this subject to you. And if giving the tochacha by a man is bound to fall on deaf ears, then it should not be given. And this would fall in line with what I said. Tochacha should only be given if there is a possibility of it making an impact. This is a prerequisite of tochacha itself.
BTW I’m not certain which Sefer you are referring to, but I think Rav Pesach Falk’s tznius Sefer is a wonderful addition to every Bas Yisroel’s library, and there is much to be learned from it. Our Rabbonim have been specifying the boundaries of dress from time immemorial.