I am working on a story in which the characters must solve a mystery. It's a "team-building exercise" at school, in which they must work together. They are graded on how well they work together and whether they are able to solve the problem.
The exercise is a scavenger hunt where objects are hidden around the school. The teams must use their first clue to find an object which will serve as the second clue, etc. until they find the last clue and turn it in.
The problem is, I have no idea where to start. I can't think of good clues. How should I go about creating clues that will be fascinating and challenging?