Not sure what the assignment is, but the main thing that stands out to me is the overall width at the widest part of the skull is off. Steve Lenze posted a great image, but another important view is the skull from the top. Try to get the widest part to match this top view and Steve's image from the front. The rest will make more sense. Also, the mouth region is more like a squashed sphere.

Hey zato, Sorry to hear your teacher isn't much help with your assignment. My best suggestion is to look at the skull for your basic shapes and planes, then look at a fleshed out head to see the surface details. I went ahead and did a quick sketch to show you some of the things that I think could help you start to see what the planes and shapes of the head are doing. I hope it helps :)