You've made a ton of progress!

 What do you mean with inversing the shadow? That I switch the colours, or that the shadow should be cast on the blackboard instead of underneath?

I guess an argument can be made that the shadow under the blueprint is the original color of the board. Kinda like what happens to construction paper that never sees daylight,  but I agree that the bottom left of the chalkboard (our right) needs its shadow cast reversed. Otherwise this is a good piece. I might add a lil shadow downward left (our right) from blueprint fold.

The thing that's now bothering me is that there is no defined light source. The light on the sponge is coming from directly in front, the corner of the blueprints makes it look like the light is comming from the bottom left due to the dark shadow on the wallpaper while the frame on the blackboard seems to have the light coming from every direction.

Looks great