Never been to Costa Rica, but I can attest to the quality of some schools in Quetzaltenango, Guatemala although my info. is a little dated (ca. 2001)
CBA- Centro Bilingua Amerindia: small school (10 ish students at a time) good staff; if it's still around. The owner, Hugo, is a heck of a nice guy who would give you the shirt off his back, if asked. The typical format is one-on-one tutoring with a native speaker who will tailor his/her lessons to your needs--if you want 4 hours of grammar a day, they can do that. If you want to go to the cafe and chat, they'll do that too.
Casa Xelaju--much larger spanish school (sometimes 80 students) quality of the teachers varies from great to just so-so.
Again, this info. is dated, and I'm not so sure what the security situation is like these days in Guate.