Hi, that's a nice amount of work, I like your figures
The heads also are looking nice and mostly well constructed
I have some feedback for the heads, but take it with a grain of salt since I haven't reached that part of the program or studied heads in a while.
First I'm just curious why some of them are upside down ? the model was hanging or something ? x) or is it to have a new look on them ?
Then the front facing head on the last page, you drew an ellipse for the center line when it should have been a line if it is full front and symetric. Also the same one, don't forget to erase the side of the base circle at the temples
If the head is tilted up or down, the ellipses of the hair line and nose should be tilted accordingly (especially in the last page). The proportions end up skewed.
On the first two pages the center line is sometimes not in the center for the 3/4 views
And finally I feel the vertical part of the jaw is a bit too short and too vertical, it should be slightly towards the front
For the construction practice, for the cylinders you can curve the wrapping lines more at the edges to improve the 3D feel, like you did for the spheres.
Hope that helps, it's a lot of text but not big problems to correct, you're on the right track!