Relationships in the context of school can be very difficult, sometimes, it seems.
Having two children, and having experienced the same things with both (and having discussed this with other parents), it seems quite normal that young children change their friendships on a regular basis. A best friend seems to be a very transitory concept at this age, with 'best' friends sometimes changing on a weekly basis. They can be pretty nasty to one another, too. Once someone is out of favour, they are often ostracised from the whole group. But then, of course, this changes again in a few days. I understand that this is all connected with learning about relationships and friendship. Getting a grip on empathy and people's feelings and loyalty. But it's quite a tough learning process.
It can be tough for parents too, as you spend a good deal of your time comforting your upset offspring and, on other occasions, gently explaining to them that, perhaps, they didn't handle that situation with their friend as sensitively as they might have done. Some parents, though, take things to extremes, snubbing other parents in the playground whose children are no longer friends with their children. This seems a pity. While there's a developmental reason for kids behaving like this, their parents should surely be well past the developmental stage. Shouldn't we be setting a good example for our children, rather than a bad one?