If a specific story doesn’t come to you, start by writing a description of what you see and move forward from there. Where is this school? What time of year is it? Who are these students? Why would someone photograph them? What class are they in? How well do they know each other? What isn’t seen in this photograph? What is their teacher like? How do you imagine these students act? What social group do they belong to? What are they like when they’re alone? What is their home life like? You don’t have to answer all of these questions, or any of them, for that matter, but they can be good jumping off points for when you get stuck.
Remember: Even if all you come up with for now is a description, keep it and come back to it later. If your muse is like mine then it most likely enjoys giving you puzzle pieces that need to be fit together over time rather than the whole story all at once. You never know what will connect your pieces together, so don’t trash something just because it doesn’t seem to go anywhere right away.