There’s no question that children have to learn how to relate to their peers (through 'thick and thin') as a part of growing up. That’s part of what school is all about, and the staff who work in a school help the children to do that.
There is a clear difference, though, between a child who is being systematically bullied, and a child who is falling out with friends due to natural personality clashes or imbalances of power within friendship groups. This can happen especially when children with strong characters are involved.
Children are constantly learning how to be 'grown-ups', and there is often some dramatic element to a fall-out which children with strong personalities actually enjoy at the time, regardless of the consequences, because it attracts attention. It is only after the fall-out that the consequences are realised, and then they get upset.
On drilling down to the reason for the fall-out, it can be the tiniest thing (or nothing!) but it is the children’s reactions to it that have escalated it into a problem. That's where a teacher's coaching and children's learning come in.
From a parent's perspective, it can be difficult to distinguish between true bullying and 'children being children' because the end result of these situations can look similar - that is, an upset child who may feel that others are against them. This is understandably upsetting for any parent and it can lead them to think that their child is being bullied, when in reality, the situation is a part of their child learning how to interact.
When a fall-out happens, speak to your child about the reason for the disagreement, trying to get to the basic cause of the issue. Often this is very minor, and has been escalated by the way in which the participants dealt with it. If you feel that there are unresolved parts of the problem, your first port of call is your child's teacher. You can get in touch by phone, email, or by speaking to the teacher at the start or end of the school day.