I swapped out the locker and added more background stuff to liven up the scene. Lighting is still WIP.
Now that's some progress. Although if you're still up for some critique, I still think the lockers need some beveling.
Also..the lockers on the back seem to be missing the lock mechanism? It can't be that the DoF is blurring it that much that I can't see it considering I can see the grills but not a big indented rectangle. I think I see some colour change actually so it IS probably the DoF.
1) Lighting is still flat on the main subject (the girl). I'd try with only the hdri to see how it looks, add some volumetrics and if nothing looks ok, only then start adding ghost lights.
2) The materials suck. It's not your fault, but I'd change both the brick and the wood on the windows. Especially the wood, it look like plastic. There are several sites with free and high quality brick and wood textures.
3) I don't understand that blue rectangle on the floor at the right. If it's supposed to be a rug, give it some volume, if it's paint/ floor colouring, I'd either remove it or make it bigger, it's distracting as it is.
4) The girl's dress. The way it conforms underneath her left breast is not correct and one of the biggest offenders in my opinion of how Daz conforming (not dforce) clothes look.
5) Similar to number 3. What's that black and white thing on the upper right corner? If it's a light, make it a light, possibly move it a bit in frame too to show it better, if not, remove it since it simply detracts from the scene without adding to it.
6) For this one, I should have cropped the image, but I can't now. Look at the lockers at the bottom gril. That hard edge looks weird at that place. It's easy enough to soften the edge on your software of choice.
7) Lastly and possibly the hardest of them all to fix, the place is too clean for a school. Lockers at the very least should have some dirt buildup at the inside corners. But that's too nitpicky and only really useful if you go for a photorealistic result.
Girl still looks weird to me.
But with the added people alone, this looks a lot better already, a better composition and better fitting to my expectations of a scool corridor. Fix the lighting and you'd be 95% there. The rest are just details.