How is Scout different from her classmates in To Kill a Mockingbird?

Since other answers have focused on Scout, let's look at her classmates and then contrast her with them.  

In the classroom scene in chapter two, Scout observes that most of her classmates recognize the alphabet when Miss Caroline writes it on the board because "most of the first grade had failed [it] last year." Scout, on the other hand, is able to read the alphabet, the class reader, and stock quotations from The Mobile...

Since other answers have focused on Scout, let's look at her classmates and then contrast her with them.  

In the classroom scene in chapter two, Scout observes that most of her classmates recognize the alphabet when Miss Caroline writes it on the board because "most of the first grade had failed [it] last year." Scout, on the other hand, is able to read the alphabet, the class reader, and stock quotations from The Mobile Register.

Some of Scout's classmates are defiant, like Burris Ewell; a "cootie" crawls out of his hair, and when Miss Caroline tries to send him home to bathe, he insults her with profanity.  Scout is among those who console Miss Caroline. When  Scout herself gets a correction from Miss Caroline, she receives her "whipping" and time standing in the corner without defiance or overt resentment.

While Scout's family is (by Atticus's description) "poor," she is clean and has enough to eat, unlike her classmate Walter Cunningham, who has no shoes and no lunch, until Jem invites him home to eat.