It looks too fake in my opinion. How would you recommend I go about doing this?
A building is not a burned sheet of paper https://www.google.com/search?q=burned+paper n_n look at correct reference images https://www.google.com/search?q=destroyed+building
0) Work on a high resolution image. I have the feeling you are working on a tiny image.
1) Take a look at Computer Graphics Textures websites. There are some that has some free images.
2) Mask your school walls and adapt the perspective and scale of the texture.
3) Adjust the color. Do not paint, use saturation, hue sliders or curves for example.
4) Integrate with the overall image adding corresponding shadows.
5) The same process if you need a missing part, but you need to look for a matching image.
6) If you need smoke do it very subtile. It is better if you add several layers of transparent smoke. Play with some blending modes.
7) You probably need debris on the floor. You probably need to prepare a brush. But that would be the most time consuming part.
Its easier to get photos of run down buildings then takes parts of those to composite to your image. Things like cracks and broken windows are easy composites. For more extensive damage you would need to match building size and orientation.