Can't recommend any school as I'm a native speaker, but because of that I can say you shouldn't worry that much about the whole tú vs vos thing. I don't think they teach voseo at foreign language academies, plus after a while you understand the shift and it becomes normal and logical to your ears.
Montevideo is nice yet small. 5-6 days sounds too much just for the city but you can always explore the Uruguayan countryside as well. And don't worry about the difference between Buenos Aires and Uruguayan accents, it can only be told by either Porteños or Uruguayans, even Chileans don't notice it.