Gibbs (2009), states that the assessment is as a key concept in any curriculum, in this way, analyzing the assessment practices teachers perform in the classroom will offer valuable information for teachers to improve the quality of teaching English as second language classrooms. Due to the leading role of assessment within the learning process, the current empiric project inquiries into the best assessment practices of English teachers at two different high school modalities (traditional based on competences and international baccalaureate) at a general high school in Aguascalientes. The high school English teachers´ performances represent a spectrum of practices that if considered as best practices offer a scenario of improvement in the teaching of English as a second language. Furthermore, we understand that best practices are constantly modified and inspired by the teachers´ believes.  The main objective of this research was to identify the best assessment practices and the English teachers´ beliefs in high school that are underlying those practices; at the same, they are contextualized. This is a case study of a mixed approach in which trough technics such as the survey and no participant observation, we got information to portray and describe the pedagogical context in which those best practices take place. Some of the conclusions this research drove are : we could observe that the best assessment practices are characterized by the participation of students in the assessment process and the use of information and communication technologies. Also, we concluded that if we want the students´ learning to improve, we have to change the teachers´ perspectives and believes about the teaching-learning process, consequently their practices will change.  Moreover, we present as a model of proceeding the best practices that teachers in this school perform.