Mean grade U in KNEC KCSE results mean ungraded, no humanity subject was done. KNEC requires that each student must do a humanity subject. Candidates who did not do any humanity subject are awarded mean grade U.
The consequence of getting grade U is that you cannot join University or College. It is as good as repeating.
You should blame your school for not following KNEC requirements during registration or the student for not sitting for any humanity subject if the school had registered him.