What can we do about private school nightmare?

My 13-year-old son (7th grade) is in small private school. He is well behaved, gets straight A's, and never has caused a single problem. He has several outrageous boys in his class who are constantly in trouble and have been bullying my son for the past three years (psychological). (Three boys have left the school because of these boys.) The boys have ruined my son's reputation by constantly making fun of him in front of the others. My son now suffers from anxiety and we have seen his self esteem fall drastically. The school has refused to do anything since they don't 'see' it....even though these boys are constantly in detention for other things, etc. They try to act as if my son has blown things out of proportion, have insisted my son sit with these boys at lunch and 'make up', have set a narrative with the teachers that my son is 'too sensitive and needs to grow a thicker skin'. The teachers have lectured my son and made a big deal out of this rather than addressing the problem boys. Basically, they do not want to expel the bullies because they don't want to lose tuition money. They lie about the boys being a problem even though many of the decent kids' parents have complained about them. One of the boys has started numerous fights, brought a switchblade to school, and writes stories about how he wants to be a serial killer and kill women with lead pipes. My son was forced to read his 'novel' about killing women as an English project. My children we sent out on Earth day to pick up the school grounds. They were made to clean up beer bottles, cigarette butts, and found numerous condoms. (Teenagers party on the grounds at night.) We find this absolutely appalling and cannot believe children we forced to do the job worse than a janitor. Further, we have now chosen to take both of our children out of the school for the next school year.