Why are they so dark? You are majorly underexposing, by like, several stops. I mean in the first image I get that you might be trying for a silhouette (still underexposed even then), but the other two, there seems no obvious intent for it.
I agree you should go try to take some real photos with it, like with artistic intent, not just "my classroom for testing purposes" and expose longer and post some more.
I also suggest playing with much wider angle shots. The main reason most people use pinholes in LF is because of the ridiculously wide angle rectilinear image possibilities. You may need to use a much thinner material for your pinhole (sand it down without breaking through), to reduce vignetting in the corners. And you may need to experiment with recessed lensboards or a bag rather than a bellows to get wider possible angles, etc. But I think that's where the really cool unique worthwhile pinhole stuff usually is. Versus just a fuzzy version of a photo you could take with a normal lens at f/32.