I think the more we teach children how to deal with kids (and adults) they do not like or get along with, the better off they are for the rest of their lives.
I read threads all the time about parents wanting to switch providers, request certain teachers in school or change bus routes all because they don't like them. I always cringe because what is that teaching them?
I know you mentioned staff changes etc so I would chalk alot of behaviors up to any type of change but in regards to his "friend E" I would spend time talking with your son to encourage him to find ways to not only defend himself but how to stick up for himself and deal with those he does not like so much or play that well with.
I tell all my DCK's they do not have to like each other and they are free to decide who they will and won't play with but they have to be respectful and kind when doing so.
So again, I'd find the lesson in this and work on that. Having the skills to get along with many other types of people will benefit your son his whole life.
