Du bist die Aufgabe. Kein Schüler weit und breit.
You are the task. No pupil far and wide. [Kaiser/Wilkins]
You are the exercise, the task. No student far and wide. [Hofmann]
Aufgabe can mean duty, assignment, or problem, as well as task or exercise. It may be that the reference to a student in the second part conditions the translation of the first toward a meaning more like homework.
If a lesson is meant, what is it preparing you for?
Kafka uses the intimate du in this one. Is he addressing himself?
You are the problem. This is true of the main characters in many of Kafka's most important works. Their problems are not distinct from themselves. Even Josef K.'s problem cannot really be described as a misfortune that falls on him from outside, and he is not without a role to play in the determination of his fate. And, in The Castle, K. brings everything on himself.
No student. Only teacher?