Middling marks are grades that are neither particularly good nor bad. The crossword solutions for the term 'middling marks' are CEES and DNUK. CEES stands for 'C-E-E-S', which is a grade given to students who have achieved an average score on an exam or assignment. DNUK stands for 'D-N-U-K', which is a grade given to students who have achieved a slightly lower than average score on an exam or assignment.