De-class refers to removing the classification of something as confidential or secret. The antonyms of de-class are, therefore, words that refer to the process of classifying something or keeping something confidential, such as classify, label, grade, rank or mark. Classifying is the act of assigning a category or grade to something, while labeling is the process of identifying and naming something. Ranking refers to assigning a position to something relative to others, and marking is the act of labeling or putting something in writing to indicate its importance or purpose. In short, the antonyms of de-class are words that involve grading, labeling or keeping something confidential.