Classwork

In this case, the answer is "classwork." In general, with respect to compound words, the best rule is to consult a dictionary. There are no reliable rules, and the language evolves. There is a tendency for compound words to begin by being hyphenated and then lose the hyphen when they really become part of the language. There is also a tendency to use less and less punctuation. Therefore, you need to be careful because if you are reading older books, you will often see hyphenated compound words that are no longer hyphenated any more.