This is the key. Teach your children that if you are uncomfortable with something your peers are doing
DO NOT JOIN IN
Tell your children to make the instigators know that you will not stand for it, make them aware that it is not socially acceptable and make them feel small and humiliated for trying to belittle and dehumanise someone else. Identify the people that see reluctant acceptance and tolerance as agreement and encouragement and have their behaviour checked before they get too full of themselves, too strong and cocky to accept anothers viewpoint.
Teenage boys are terrible for feeling the need to impress one another, teach them better ways of doing it