My partner Fernando Romero offers classes at our apartment in Belgrano (although the map says we're technically in Palermo). Subte Olleros. About 15 mins from Palermo Soho. He's posted in the classified section.
Marta Vigués is the best language coach I know. She's worked with a number of friends, including one who continues lessons via Skype from Paris. Delightful lady who speaks a half-dozen languages, including English (better than I do).
She's near Santa Fe and Scalabrini. 4832-2964 or 15-5567-7181
She is a great teacher who cares deeply for her students. Didn't even feel like a school really, more like a cool place to hang out and learn Spanish. Lots of cool students there and made quite a few friends.
She is located on Sante Fe right next to Alto Palermo.