Is Panzerfahren only for girls? In the anime series, all the Panzerfahren participants are girls. Are there any guys who also participate in Panzerfahren? And if I recall correctly, all schools that participate in Panzerfahren are also all-girl schools. Is there any rule of Panzerfahren that all participants must be girls?
In the all-girl schools, it is compulsory that all students select a particular extracurricular activity to do.

These activities that are offered to the girls are ones that are symbols of femininity and lady-likeness. At the start of the series, the girls have to choose between flower arranging, tea ceremonies and tankery. All equally feminine subjects (>_> in the Girls Und Panzer universe at least)

Because tankery is seen as a feminine sport, there are not many male players. There are possibly some, but the overwhelming majority is female as many areas would not have enough interested guys to form a team. 

If you tried to form a flower arranging club in an all male school, you would probably find there would be few members - This is the case with tankery also.