A great mystery I read was "Bad Girls Don't Die" by Katie Alender. This book was pretty creepy. It was a mystery while also going into paranormal stuff as well.
If I were to write a mystery I really don't know what it would be about. The one I wrote for Language Arts was pretty good although, it didn't represent me well. I would do a mystery that relates to me/my life. Something maybe at my dance studio where everyone's left tap shoe goes misiing for some weird reason. Or a mystery at the barn where I board my horses. Something where the horses change size from draft size to mini horse size and we don't know why.