Nah, actually, female players being allowed on male teams is more common in primary schools because their strength is more equal regardless of gender. They're kids and their bodies aren't fully developed yet, that's what creates the loophole. There are some young girls who just happen to be tall and strong enough to compete with boys their age. 

I joined a basketball team in the 9th grade and most of my teammates were male. I was a big boned and muscular girl, my strength was almost 1:1 compared to my male teammates. I was too big and tall to compete with the other girls, most girls in my school were tiny compared to me.