Different theories over the past century and nowadays have been developed to clarify how a second language is acquired by learners. It is not necessary to spend so much time, nor effort, to highlight the great importance SLA these days have; but this acquisition seems to be an extremely easy or difficult task depending on the variety of factors and circumstances which determine it. As a result, it is paramount to analyze which is the most appropriate model talking about SLA; model that will allow us to specify the critical variables, know the process of learning-teaching L2 and elaborate strategies to manage this process through the most effective road. Many of these models are still in discussion, others outdated and some of them, still being renewed due to social factors. And we are dealing with this from a practical point of view: SLA in a Spanish High-School class with teenagers from 13 to 16 years old.