Many universities, at least in Europe, allow you to sign up to individual modules/exams. This is usually used by people who want to get ahead and study stuff be

Many universities, at least in Europe, allow you to sign up to individual modules/exams. This is usually used by people who want to get ahead and study stuff before they actually sign up to a degree, but anyone can do it. At that point, the author would have been a student of that class like any other.