"En kurs" can mean either of two things: a longer course like a college course that spans over quite some time, or a one-time thing like a four-hour corporate training opportunity.
In the latter case, "kurs" does mean "class". But in the first case, a "kurs" is comprised of a long series of individual "classes". Hence, it's not always 1:1 - but the word does have both senses.
That said though, "kursen" does mean "the class" in the latter case.