It's cool seeing all this practice!
I did a couple of draw overs to help with the structure. The main things I went over are the placement of each facial feature and the overall form of the head.
For the first head, remember there's space under the chin, so try to add this when you do extreme angles. Also, be careful when you draw the ellipses. Make sure they're smooth and the cross-hairs are perpendicular to each other. Once you get the cross-hairs right, make sure the mid-line of the face is parallel to the vertical cross-hair.
I think it would be good to use straighter lines for the jaw because they're more general and help capture the basic form of the head. The jaws here look very specific and a little distracting since the other lines and shapes are so simple.
Hope this helps!