User blog comment:ChishioKunrin/Is Qrow a weapons teacher?/@comment-3257939-20151022135712

So basically, it's like High school and College. Combat academy teach you the basics while Huntsmen Academies are one of the options you can chose from to get a career. You learn all you can about Grimm, Teamwork and Missions. You become a specialist in fighting Grimm.

There's also the fact that Signal is located in Patch so it makes sense that the two sisters chose to learn from the school closest to home not only in distance but also because their family works there as teachers. And Ruby said that she was complete garbage before Qrow which means that Taiyang, for some reason, didn't train her and that could extend to Yang too thus leaving Qrow as the only person that can mentor the girls.

Here's another thing for that Slightly Related Side Speculation though it's a little unrelated:

Back then, instead of fighting Grimm, humanity was fighting itself. All the knowledge about Grimm and fighting was spread all over the world with people refusing to share it. Emerald's words gain a new light with this info: Tall buildings, diverse culture... Does that mean, out of all Kingdoms, Vale is the one who shares more cultures with the rest of the world? Makes sense considering it's in the middle of the entire planet.