how does a book become a classic?

The publisher gets a contract with all the schools and millions of students are forced to read the book. 📖👀

As far as the books we read in school, often times they're chosen because they're mass-produced and cheap for the district. That's why everyone has to read To Kill a Mockingbird or The Great Gatsby no matter what part of the US they grew up in.

Another part of the equation is that teachers read the same books growing up and decided it would make great teaching material. You look back at something like The Scarlet Letter and realize it beats you over the head repeatedly with symbolism. Or look at Lord of the Flies and realize how the characters represent specific parts of the human psyche. They may or may not be great books, but they are great for teaching literature.

Another reason may simply be cultural. They show us glimpses into American history and teach us what life was like during The Grapes of Wrath or Uncle Tom's Cabin. It shows people the parallels and contrasts between life today and life all those years ago.

I'm sure there are other reasons, but those are definitely major factors

When it is still popular several generations later

Usually because it's a good story with meaning that isn't limited to place or time, a story that speaks to many readers.

Sometimes it's because it demonstrates change from one literary era to another.

And once in a while it's just because literary gurus say so.

After time passes and someone famous deifies it or if they make it a part of curriculum

Isn’t it after the author dies

The amount of sales

Time and interest ?

If it's good or not.