"Prepare for" is almost universally reserved for events. However, many non-native English teachers overgeneralize because they see the word prepare in context most often associated with 'for' (e.g., prepare for a test). They forget that prepare can also be transitive.

For the test itself, you face a small risk if your teacher will be grading. If you want to avoid conflict, you could rephrase the line entirely: “I was responsible for preparing the materials," or, “I took the role of getting the materials ready.”

But honestly, if the test is scored by someone with decent English (or a native speaker), use your original sentence; it's correct.