This matter is serious. You can’t take it lightly. As this incident has happened inside the school. So, the school is responsible for this. You can write a letter directly to the principal. You can ask the principal to take severe action against those boys. Also, try to find out what was the reason they did this to your brother. You can ask for the details and addresses of those boys. So, that you can personally go to their houses and talk/complain to their parents. They need to be suspended. You need to take this seriously.