My friends hate each other.
Why do your friends hate each other?
I'm not sure they all used to get along well because we've known each other since 5th grade.
If your friends are not getting along together, try to bring everyone together and discuss the issues that are damaging the friendships. If you can't work out the differences, then you may have to spend time with them individually. You shouldn't force people together that don't like each other.
I thought my friends would never associate with each other again.
That may be the case, but you won't know until you try. Either way with this plan you have a way to deal with either outcome.