That Yahoo search is doing better than Dano did, but it is still nearly 2000 results. As in most things relating to CR, you really don't have to go far from CRT to get the information. You just have to learn how to use the search engine here properly. Did Dano ever think of trying a search of the words "Spanish" AND "Schools" (remember to check "Search for all terms")? That results in a much more manageable 39 results.
Here is one particularly good thread:
http://www.costaricaticas.com/phpBB2/viewtopic.php?t=9231&highlight=spanish+school
Oh yeah, and my own personal recommendation would be CRLA (where I went). It is located near the new Hotel M, which, though very convenient to CRLA, spending a full month at a regular hotel would probably break the bank. You'll also find info in those threads on good places to stay nearby for an extended one month visit.