Short version: Lord of the Flies at Catholic school, done by a master.
Longer version: I really wish I had gotten to read this in high school. I didn't go to a Catholic school, but I did go to "catechism" every Wednesday at the local church school, and I also lived in the author's conservative Massachusetts hometown. The events in this book are all too believable.
This is the story of Jerry, a boy who has recently lost his mother to cancer. His dad is a pharmacist who works all hours, so Jerry is on his own a lot. Jerry starts at 9th grade at a new (parochial) school, tries out for the football team, starts to make friends, etc., and is doing pretty well. He soon learns about the existence of the Vigils, the ruling clique, who assign other students to pull pranks, commit vandalism, etc. Nobody refuses an assignment from the Vigils.
One of the teachers has gotten the school into financial trouble, so now each boy at the school is tasked with selling 50 boxes of chocolate, double the usual number, within a very short time. As a prank, the Vigils tell Jerry to refuse selling for 10 days, which does not go well for him. But it's what happens after that that sets the rest of the book in motion.
Cormier does a wonderful job of getting across what it means to lose a parent. He also succeeds at creating the claustrophobic atmosphere at the school, showing the corruption and brutality of its rulers (teachers and students), and describing Jerry's growing isolation as his friends fade away, one by one. You can't help but root (and fear) for Jerry as he stubbornly does what he thinks is right, against all the norms of the time and place.
Every high school should teach this book.