I think the best mystery story I've read is the "Speckled Band." I think the author did well was making the reader think hard about what it might be.
On the last day of school every kid was excited for summer, they had one more minuet left till the bell would ring rriinng. Every kid ran through the hall bursting with joy until the got to the doors and noticed they couldn't get out and the windows were locked and can't break. Some students started to freak out but they noticed the teachers had stayed strangelly calm everyone was curouse to what was going on.+