The objectives of this study were to (a) verify the validity and reliability of a scale to assess skill levels to learn content knowledge (SLACK) and (b) develop percentile norms to assess the SLACK of adolescent students. The study included 2270 students (1134 males and 1136 females) ranging in age from 10.0 to 18.9 years old. A scale was used to measure the SLACK. The results showed high values of validity (saturation between 0.32 and 0.54) and with internal consistency from 0.83 and 0.84. The LMS method made it possible to create percentiles p10, p50, and p90 in order to classify the (SLACK) based on category, age, and sex. The results suggested the use and application of the scale with the school education system by using the percentiles to diagnosis, classify, and monitor the skills for learning knowledge validly and reliably.