This study had the objective to analyze the mental processes in terms of the thinking abilities and the cognitive functions as well as some levels of text processing that distinguish (differentiate) good and poor reader students from High School. The analysis was based on the theories: The Structural Cognitive Modifiability of Reuven Feuerstein and the Text Processing of W. Kintsch and T.A Van Dick. The cognitive demands of solution from each one of the questions of a standardized reading test were analyzed as mentioned before. The following cognitive processes were identified and organized: Classification, Analysis-Synthesis, Logic inference and Hypothetical reasoning. The sample was conformed by 258 students (males and females) from second to fifth grades of private high schools, class B, and different techniques of data analysis were applied such as: 1) ANOVA design to determine significant differences between the performance of good and poor readers in items corresponding to each one of the cognitive processes by grade of study; 2) Cumulative percentage of the correct answer frequency of the good, the average and the poor readers by category of abilities and grades. In a first analysis level, the quantitative results showed significant differences between the scores of some items of each category of thinking abilities between good and poor readers. These differences were increasing in the last grades, in Analysis-Synthesis and Logic Inference. Difficulties in hypothetical reasoning were maintained in all grades in the same test item. In a more detailed quantitative and qualitative analysis, explanations of the differences were attempted in relation of the thinking abilities and the mental functions as well as the text processing aimed to show the differences in reading achievement and also the difficulties of each group of students.