If you are doing Marc's course on anatomy and I haven't taken it so I cannot provide any corrections from what your teacher may already be providing. I dont want to give you contradicting information.
I will say though that you have a lot of "petting" in your line work and not confidant strokes ( I address that below) and you are somewhat drawing the "symbols" for anatomy like letters for the alphabet instead of thinking of things in 3 dimensional shape/construction forms like adamjwass already mentioned.
But I can apply some Andrew Loomis Perspective over your head construction. I'll find errors I've made and replace the image here over the next week probably. Cheers.
Also you can find all Andrew Loomis Books on Archive.org placed there by various libraries.