"Spanish Schools"
Hey Brita, I have taken classes at the well known "Instituto Cervantes" in Dublin and also in "Academia Uruguay" here, but I now have a great private tutor that I would highly recommend.
She's Uruguayan but lived in Spain and England for many years. She's very professional and she teaches both Spanish and English in multiple styles (basic beginner, advanced, conversational, business langauge etc). Her name is Rosa and her cell phone is 096236034. She either come to you or you can go to her (she lives in La Blanqueada, just north of the Centenario stadium).
Academia Uruguay is also pretty good and professional, but the prices are fairly high if you plan to study on a continual basis. Also, its been a year since I studied there and I think many of the teachers have changed so I can't vouch for as I once did because I'm not sure, you could ask new arrivals Scott and Heather, I believe they're giving it a go.
There is a school is Pocitos called La Herradura, I would avoid it like the plague. Any reports I have ever heard of it were poor - they're unprofessional and they mix people of different levels together so nobody really progresses, its more aimed at young travellers than older professionals.
There's also International House on Avenida Brasil, they seem pretty pro, but no personal experience with them.
Well, hope that helps :)