Каждый из учеников получил книгу - means everybody got a book
Каждый из учеников получил по (одной) книге - it would mean "everybody got one book"
These sentences are interchangeable
you can say
Каждый из учеников получил книги - everybody got books
But you cannot say
Каждый из учеников получил по книги - it's incorrect. This construction can only be used for single objects

However
Получить по морде - Means "get beaten" or "get punched to the face"
Получить по заслугам - means "get what one deserves" (usually in negative meaning)
Получить по первое число - it's an idiom. Means "get punished"

But
Получить по запросу - Means "get something by request"

I hope you are not more confused than before