When I looked online about a month ago for Spanish schools in Guatemala there were at least two ranking/review sites. There will likely be not only a surcharge for booking online, but the base price will also be inflated. If you're uncomfortable showing up without a reservation, don't book more than a week -- you can pay for the rest of however long you wish your lessons to be at what is hopefully a reduced price (or swap schools). I've heard praise for those in Xela. San Pedro La Laguna on Lago Atitlan is a popular destination to learn Spanish, but I found there to be more English (tourists/expats) than Spanish in the area where the schools are, though some enjoy this after 4-6 hours of tutoring.
Note that motorcycle parking is scarce in Antigua, but many hostels/hotels can put yours in their lobby, courtyard, bar, or something. (El Gato Negro / The Black Cat put my KLR in the bar entrance, but it was a pain and had to be done after they closed at 11PM; the Dionisio put mine in their lobby, though there's only room for two -- one, really, as the current nightshift guy takes up a space with his -- and I had to remove the panniers.)