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.
According to the Girls Und Panzer wiki, Panzerfahren (Sensha-do) is:
a martial arts that involves tanks as weapons. This martial arts are just exclusive for high school girls to develop their self-esteem as young women.
Like you mentioned, all the participating schools are all-girls schools. The rules themselves don't seem to limit the competitions to only girls, but the martial art itself seems to be only for girls.
Episode 1, Tankery, Here It Comes!
At 17 minutes (17:05 to 17:10) after the 'Introduction to Tankery' film has been shown. Miho Nishizumi, Saori Takebe & Hana Isuzu are in the cafeteria. Unknown students are chatting in the background, about Tankery.
One says, "They say its perfect training for women."
Another student excitedly says, "I never heard of men doing Tankery. Guys and tanks seem kind of mismatched to me."
(I only noticed this revealing bit of background info on my 4th rewatch!)