I think the grass does look a little too big compared to the rest of the scene but it's not bad for a start. I like how you at least used different colors for the grass as it makes it look a bit more realistic.
If you could have some debris outside the scene, that may help too. You've got nice walls, etc. but a thought that could cross someones mind is, "Where's the pieces of the walls that crumbled and broke off?" Anyway, if you get some extra time before the comp ends, maybe you could put a few pieces on the ground outside the building and partially sticking up through the grass.
Don't be satisfied with what you can do but rather strive to do the things you can't do!