I ran into an article on How you can help build bottle schools for kids in Guatemala. While this organization only does these projects in El Salvador and Guatemala, the idea looks interesting. If it works on two of the CA-4 countries, it should work in the other two.
I am not very excited about the volunteer opportunity aspect of the particular project ($1200 plus your own air travel for a week mostly doing work) but the idea of building the schools from close to nothing is interesting. I would be interested in more details on the actual construction particularly focusing on cost and materials savings.