San Pedro La Laguna has a concentration of Spanish schools that makes it one of the cheapest places in Guatemala, and therefore Central America, to study Spanish intensively. The standard arrangement is four or five hours of one-on-one instruction per day, typically combined with a homestay with a local family. Prices undercut Antigua significantly.
The setting helps: after morning classes you are on the lake for the afternoon. Most schools can accommodate short stays of a few days or longer commitments of several weeks. Quality varies, so reading recent traveller reviews is worthwhile before committing. The teachers are native speakers, typically from Tz'utujil Maya backgrounds, giving the language instruction an additional cultural dimension beyond the textbook.