Many schools are available. We went with Celas Maya which has a couple locations across Guatemala. For more, go to their website: celasmaya.edu.gt
We spent two weeks at their Quetzaltenango school, locally referred to as "Xela" and pronounced as "Shela". From here, we travelled by bus to the school's second location at San Pedro La Laguna on Lake Atitlan for a third week of classes. The town of Xela had a more authentic "working" city feel, whereas San Pedro La Laguna was smack in the middle of the tourist trail. So, it depends on what you are looking for post-class.
Teachers were attentive and clearly explained the theory. With five hours of one-on-one time daily (or less, if you choose), there was heaps of time to practice and perfect my pronunciation and comprehension. Intense, but worth the effort!