I found these reviews useful:
123teachme - http://www.123teachme.com/language_schools/. This covers schools throughout Central America.
Survey of Spanish Schools in Guatemala - http://www.guatemala365.com/ (I reserved my stay at Probigua through this website. The web manager later emailed me for feedback on my experience there.
The Spanish Schools of Antigua Guatemala, by McCormick - http://www.geocities.com/guatemalanspanishschools/spanishschoolsantigua.html
Guatemala Spanish Schools - http://www.guatemalaweb.com/SpanishSchools.htm
Top 10 Spanish Schools for Waves, Wilderness and Buena Onda - http://thetravelersnotebook.com/top-10-lists/top-10-spanish-schools-for-waves-wilderness-and-buena-onda
Also look at the reviews in guidebooks, such as Lonely Planet (Guatemala, Honduras, Nicaragua & El Salvador).
There are numerous other websites and blogs. Your research will be limited only by your available time!