Biblio Talks is a language school inside the NGO Biblioworks with four profesional teachers. You can have individual or group classes. As the language school is till quite new (they only started last November) they offer a discount to the first 20 students.
All proceeds go to the NGO as Biblio Talks is a Non-Profit Organization. Furthermore the school can arrange you a placement as a volunteer directly. All their projects are in the countryside and would be an extraordinary experience for you to practice your Spanish and see how to live in a typical village. (See their propfile at the volunteer work section of Bolivia-Online).