This probably varies quite a lot, but whenever I fail such tests[1], the first person they contact has always been the employer/school/team, not me. Usually, the business is discrete enough to privately reach out and allow me to explain myself. More than once, however, I would only find out about my failure and imminent expulsion after-the-fact via letter/email -- I've always been able to fight my way out of these situations, but often without my personal reputation intact.

[1]: Which I always do, as a person who has been prescribed Adderall since childhood[2]

[2]: Yes, the diagnosis is legitimate. It was diagnosed by a trusted family pediatrician during a regular check-up and was not solicited by my parents[3]

[3]: Yes, it is exhausting to explain myself, my parents, and my medical history like this whenever the topic comes up. Woe is me, woe is me, etc.