I think there are several arenas where girls and women benefit when men are excluded; secondary education isn't the only one. A few others to mention:
- Sports and physical competition. Excluding men enables women to compete on their own plane, set world records, etc.
- Restrooms, locker rooms, and the like. Excluding men protects women from unwanted advances, privacy invasions, etc. I know this isn't a popular opinion, but the primary benefit of segregated restrooms is that women do not have to be afraid to go to the bathroom. Besides sanitation, this is one of the greatest benefits to society of having restrooms in the first place. (In areas with no restrooms, such as rural India, women often have to risk being molested when trying to relieve themselves.)
Another unpopular opinion: desegregation efforts in the above areas (including those motivated by wanting to achieve "gender equality" or allow transgender women into areas designated for women only) will result in harm to women.