Knowledge on back care is important to prevent back problems. The purpose of this study was to a) describe back pain and knowledge on back care related to physical activity in adolescents and b) determine the association between these two factors. This was a cross-sectional study. A total of 276 students (aged from 12 to 17 years old), from three Spanish public secondary schools, had to complete a survey about back pain and another one about their knowledge on back care related to physical activity. The results showed a mean score of 2.54 ± 1.85 (out of 10), and 90.58% of students failed the knowledge test. Those who suffered from back pain in the week before scored higher (2.91 ± 1.66) than non-sufferers (2.42 ± 1.90), with statistically significant differences (Z = −2.109; p = 0.035; r = 0.127), although with no academically relevant differences. Finally, despite the relationship between a higher mean score in the knowledge test and back pain within the previous week, it has been found that there was not a significant association because most of the students got very low scores in the knowledge test. It also shows an important lack of knowledge about physical activity for back care in adolescents.