If you pass a construction site in Mexico City, Colima, Guerrero, Guadalajara, Puerto Vallarta or pretty much any pueblo or ciudad, you may just hear a different kind of sound. It’s not the usual thud of masonry or the whirr of a concrete mixer, but the gentle murmur of voices counting and sounding out syllables. It’s the sound of adults rediscovering literacy and learning. Because under a patched...