The age from 12 to 15 is adolescence, which Rousseau thinks is "the period of labor, education and study". Because when the child reaches the age of 12, he is physically strong, has developed senses, and has the ability and habit of working independently. "He can carry out intellectual education and labor education." The purpose of education in this period is to develop students' intelligence, cultivate students' interest in science and study scientific methods.

In terms of teaching content, Rousseau believes that knowledge that is beneficial, necessary, correct and in line with children's understanding ability should be given priority. He attaches great importance to the study of astronomy, geography, physics, geometry and practical subjects such as reading, writing and arithmetic. He completely rejected theology and humanities, thinking that these subjects could not be studied until 15 years old.