Here are some books, and movies too, about people from different social groups fighting or kissing, fighting then kissing, or kissing then fighting, etc.
Fighting, kissing, more fighting: West Side Story. Talk about a rumble. This classic movie is a modern version of Romeo and Juliet. The Sharks and the Jets are sworn enemies. Meanwhile, Tony (Jet) and Maria (sister of a Shark) manage to fall in love. Lots of fighting and tragedy ensue. Because the movie is a musical, there is also lots of singing, dancing, and really good outfits.
Again, fighting, kissing, fighting: The original. Romeo and Juliet. What was up with those Montegues and the Capulets, anyway?
Fighting, kissing, fighting, kissing, fighting, poor kid loves rich kid, kissing, fighting, and so on. Life: An Exploded Diagram. Love story, meanwhile, the world is at war. I can only begin to tell you how much attention this book has gotten among the librarian set. It is much loved. However, it is a pretty epic (read: long and dense) story, so if you like tales that span multiple generations and world events, this is your moment!
Poor kid plays rich kid: Also Known as Rowan Pohi. As a joke, Bobby and his friends invent a person (Rowan Pohi) and apply to a fancy prep school under his name. When Rowan gets accepted, Bobby decides to live the life of the upper crust for a change. Balancing his double life gets a tricky, as you can imagine it might.
(Perhaps not really) Rich kid redistributes wealth to poor kids: Pretty Crooked. Willa, who moves an awful lot, finds herself in an exclusive prep school, where the rich kids bully the scholarship students. Willa takes revenge by stealing from the "glitterati," and passing it on to the poor kids. Very Robin Hood.
And there we have it. Happy Mash Day.