It is referring to the group of streets that are numbered 75-79. So the exact location is not known, it could be 75th St, it could be 78th St, but the author doesn't know for sure. They just know the general location - the upper seventies, somewhere between 75th and 79th.

It refers to the address. New York City streets are laid out in a grid with numbered streets and avenues. Her school is on the Upper East Side and her street number is 70-something. http://www.amny.com/real-estate/city-living/manhattan/city-living-upper-east-side-in-the-70s-1.8019983

It's referring to street numbers. The upper seventies (75, 76, 77, etc.) and other areas of the Upper West Side are a suburb area of New York.