It's just normal kid's stuff, isn't it ? 😕
OK, they used religion as an excuse, but it could have been anything, mods, rockers, punks, football, gangs, etc
War occasionally broke out between my school and another school when I was a kid. There was never any real reason for it, apart from the fact that they were "another school" and apparently a bit "soft". 'Loyalty' and tribalism kicked in, and if one kid from my school got rolled, then revenge would be sought.
I wouldn't worry about – no one got hurt, and I reckon the teachers had the right attitude. Maybe different if someone had got hurt.