I wrote two for you :)
This mystery is who killed one of the characters teachers, who was very dear to them. The character leaves the classroom to go to the bathroom, and when they come back their teacher was dead on the floor with stab wounds. They look around the classroom, who was in pure shock. They scan the classrom to find that 5 students have fleed. They wait until the next day at school to see the 5 students. They pull the students aside at lunch break, and ask them questions about the teacher and what they were doing during all of that. One students answers, "I was going to get some water." Another answers, "I was going to the bathroom." Two answer, "I was going to return my book to the library." And the last one answers, "I was going to get a teacher." They immediately knew who'd killed the teacher, because they went to the bathroom themselves and no one else was there except for them.
Here's a more lighthearted one, this mystery is who broke the lamp in the kitchen. The character leaves the house to go to school, and when they come back, they find that the lamp in the kitchen was broken. They call over their family. "Who broke the lamp?" they ask. They all point fingers at each other. Person A points at Person C, the girl. Person B points at Person A, the boy. Person C points at Person B, the baby. The character immediately knew who did it because the baby stayed in its crib the whole day and couldnt get out. They were also too weak to break the lamp. Person C has broken the lamp.
