Pedro, you did the right thing by connecting with other parents. School administrators generally know how to handle one case, one family, at a time, and are even willing to lose one here and there to "protect" the school. Once you have three families or more, watch the dynamics change. I'm sure you will be able to deal with the boys, and you can probably make the school commit to safety. Tell the school that you want a permanent safety committee made up of parents and students, who will invite participation from the faculty or administration. Find the most capable mother to reach out to others, the most capable father, the most trusted teacher, one outside expert, and girls from each mathayom grade. Start with girls because they have been the ones mistreated. Soon enough safety will encompass bullying, mistreatment by teachers etc. If one school gets it right, other international schools may follow, because it's a reputational thing for them.