How would you feel if your friend went into school and spoke to the teacher about some minor
squabbles that both our kids had been part of?
I found out that my friend has spoken to the teacher about my child over some minor things but didn't mention it to me at all. It's not like I'm a parent she doesn't know. We've been friends for a few years years, many days out with the kids, socialise etc together. Genuine friends not just a fellow school mum I chat to.
The teacher hasn't contacted me so was obviously satisfied it was nothing major (my child wanted to play with other kids and didn't want to do what my friends child wanted to do is the jist of what I was told).
My friend told me that my child was being mean and said she'd been to speak to the teacher about it 
I guess if it was me I would mention to my friend that there seems to be something odd going on, get the kids to talk it out then move on?! I wouldn't involve the teacher with something minor and I certainly wouldn't keep it a secret. Though I understand keeping the teacher in the loop so they can keep an eye on the situation if necessary.
Kids will be kids and sometimes they can be awkward and difficult or just fancy a change. I don't think my kids are angelic, they all have their moments, and I would be more than happy to talk about anything that might be causing upset within the friendship group. I'm quite a timid person by nature and hate confrontation, so she knows I wouldn't blow up at the suggestion something had happened with my child. My friend will also know that I don't tolerate unkindness so if it became clear my daughter had been unkind I would deal with it appropriately.
I guess it's just taken me by surprise that she didn't mention it to me until days later and announced she'd been in to see the teacher about my child 'being mean' ?!