Here we give an overview of the Mexican experience of a national program, begun in 1997, of gradual implementation of computational tools in the lower secondary-school classrooms (children 12-15 years-old) for mathematics and science. This project illustrates, through the benefit of long-term hindsight, the successes and difficulties of largescale massive implementation of technologies in schools. The key factors for success and for transforming school practices seem to be: adequate planning, gradual implementation, continuous training and support, and enough time (years) for assimilation and integration.