If one focuses on one subject for the entire time they are at school, they may lack the know-how necessary to be successful in their careers. For example, Pharmacists are known for being good at pharmacology. They do, however, specialize in mathematics as well as a few other branches of science because their job requires them to do so. If pharmacists did not study the chemistry needed to create medicines, they wouldn't know how to make medicine
Students should focus on one subject for a future career. That is how it used to work back in the day of apprenticeships, and the economy worked out a whole lot better back then. If they just became specialized in one field, that would allow them to spend less time in the school and more time working.