What do "dari" and "dan" mean? And what do K^{1} and K^{2} mean, with superscripts rather than subscripts?

But it appears that K_{l} means the set of students of a teacher named *l*.

We really need to see the definitions of all the sets (K, M, N); and the wording of the definition of K_{l} confuses me, as I would have thought K, not k, is a set. Did you copy the statement exactly as given to you?