I need help planning a school-wide murder mystery party?

I think this is a great idea! Maybe you could guide the groups through like they guide groups through haunted houses. Have a few people designated as 'leaders' and they take the groups from room to room or location to location. And maybe for small games or event to get the clues you could do riddles, illusions, ask questions to see who has been paying attention, or small obsticle courses. As for the clues being too easy perhaps you could put some in riddle for. Hope i helped at least a little! Good luck!