I have no firsthand knowledge of either of these 2 schools and don't know if you already knew about them, but you could check them out.
Academia de Idiomas Escazu
http://www.amerisol.com/costarica/edu/escazu/pattern.html (for which I've seen some poor to mediocre reviews) and ILERI Spanish School
http://www.ilerispanishschool.com. Frankly neither of these look that promising.
Why do you have to stay in Escazu? If I were spending a couple of grand not to mention 2 months of my time trying to learn Spanish I would put greater emphasis on going whereever they had the best program rather than shopping around based on what city it was in. There are plenty of nice places to stay and places to go in San Pedro and the Gulch is just a 5 minute bus trip away. If San Pedro is too urban for you there are other high quality schools in the central valley in nice environments if you don't limit yourself to Escazu.