This paper reports an evaluation carried out from 1999 to 2003 in 55 schools in the Basque Autonomous Community involving more than 10,000 secondary school students. The variables measured include achievement in Spanish, metacognitive skills, learning strategies and variables related to the students' family environment. The measurements were made at three different times in the last year of primary, and second and fourth years of secondary school. The main aim of this research study is to conduct a longitudinal study of the students' achievement using a measuring unit which can be used for the three measurements for all the students. The data reported in this paper focus on proficiency in the Spanish language. Data were analysed using a hierarchical linear model (HLM) with three levels: (1) the three students' measures at three different points in time; (2) individual variables related to the students; and (3) the variables related to the schools. The paper presents a model of change in achievement in Spanish by examining the influence of background variables including socioeconomic status (SES) of the family and the school, gender, and type of school (private or public). The results indicate that there is a linear increase of scores in Spanish along the school grades. D model students with Basque as the language of instruction obtain lower results than the rest of the students. Other variables such as gender, SES (family and school) and metacognition also have a significant effect.