From my experience, I did 'Of Mice and Men' in school at 13. Took some explaining to fully understand all the implications, but it was a good read. To be honest I consider this book my first baby step into leftism ("Severely neurodivergent people could live a healthy, contributive life if society cared? Society can discriminate against whole peoples who just have their own dreams and are normal nice people?! Criminals can be good caring people?!? Rich property owners are predisposed to being exploitative, abusive bastards?!?!"). Also eventually led me to read Grapes of Wrath, though probably not for most <16 year olds in the immediacy.

For younger ages I'd totally recommend Dr Seuss (despite his problematic wartime cartoons). For leftism specifically - I recall Sneetches and The Lorax (the original obviously), Sneetches calls out racism and fast fashion (and the way capitalists happily profit off sown divisions), and the Lorax obviously is a great criticism of capitalism in its entirety, he probably did others I don't recall right now.