I have specified whether my classmates are male or female in their 'Details' setup. However, the gender silhouette displayed on their profile is not the correct gender. What am I doing wrong?

It won't answer your question, but I just tested here using both joined and non-joined members. Some were not set (and showing the wrong silhouette to begin with), but all showed the correct silhouette after I changed it.

Did you set them manually, or is there a field in the upload spreadsheet which is supposed to do that - and perhaps did not?

I should mention that the silhouettes are not a factor to anyone but admins looking at profiles once you've uploaded yearbook pictures or members have a master photo.

I manually set them up when entering the names and email addresses of all classmates.
We probably won't upload yearbook pictures because we have the pages of the yearbook scanned (class of 365), and I doubt classmates will upload a master photo. We are mainly using the site to provide security over classmate information.
Thanks for your response anyway John.

All set here.