hmmm, i don't believe makeup school only teaches how to apply makeup. i'm pretty sure you learn about sanitation practices, skincare, etc. so even though you may know how to apply makeup, the courses will teach you other important things.
it's almost like saying if i can paint nails, then cosmetology school is a waste of time - i know people who do nails, and they have to learn about sanitation practices in the business.
i mean, some people may know these other important information - but if i'm getting my makeup, nails or hair done, i want it with someone who is not only artistically talented, but who i know for sure is educated in the other realm of the business. just peace of mind for me to know that they know that stuff.