When learning and practicing, you have to give a lot of thought to the true purpose of the exercise, and how it should benefit your overall artistic growth, so you know exactly what to focus on in your practices.
The reason for learning the structure and proportions of a skull, is so you can understand the underlying structure of the human head, which benefits you when you depict human heads. So when learning about the skull, you must also relate it to normal faces and heads, so you can see exactly how the bone structure affects how a human face/head looks, and you need to reference a wide range of different faces and heads, with different age groups (baby all the way to very old people), ethnicity, sex, attractive to ugly, thin to fat, and in various different lighting situations.
Memorizing details about the skull that don’t even show up once covered up by fat and muscles and skin, is nearly useless. If you need to actually depict a skull in a piece of artwork, just reference skulls. Only memorize what’s actually useful to majority of the types of images you would actually create as an artist.