Is it ethical for students to consume Adderall during university courses, if they don't have ADHD? Assume a student in the United States who has access to Adderall despite not suffering from ADHD. Would it be ethical for them to take the drug during their studies? 

Assume that the school is within the United States. When answering only the academic side of things should be taken into account – I am fully aware that taking a drug without a proper prescription is illegal.
Academia is not a competition, neither is school. It's about education and knowledge. Neither comes and goes with a few pills.

Applying the anti-doping rules from sports is therefore nonsense. What applies are the common workplace anti-drug rules.