I've been to Guatemala several times and always spend some time around Lake Atitlan. This is maybe the third most popular place to study Spanish in the country and has many language schools for foreigners, but most of the local people speak one of three Mayan languages and Spanish is their second language.
But I would be very much more interested to find out if there is a language school (or known private teacher) in San Pedro or one of the other villages than also teaches Tzotzil or one of the other local languages.