Joeys has more students playing rugby due to many reasons. The main ones being the general demographic that goes to the school, and the culture of footy there.
Firstly, I'm going to try and say this without getting into trouble, but the students at Joeys are more country boys and more traditional Australian footy-heads. Other schools are generally more diverse, thus leading to a more diverse sport selection.
In addition, the culture at Joeys is primarily around the great game of rugby and their incredible history of success. The school is pretty much known for rugby and parents who send their kids there know this.
Also there would probably be some teasing of kids who don't play footy.