I tried to get the nasal cavity too, but couldn't get the cheese cloth to stick in to my liking, so settled for another coat of latex in there.
Let dry completely before attempting to remove.
When removing with all these undercuts, I've found the best means is to begin at the back and start rolling.
Just inch it forward leaving the face for last.
Pull the face out careful not to tear the eye/nose sockets.
The whole mold tends to be awefull sticky, so I like to leave it sitting inside out for a night or two, making sure to unroll any bits we rolled up so they don't stick to each other. Some suggest using baby powder to prevent this stickiness. No real experience with that, but a little air time always seems to work.
Be back soon with instructions on how to make nice hollow plaster skulls from this mold, as well as how to color the skulls you make.