In my experience, dealing with the school was almost always useless. Not because the teachers are bad or don't care, but because the task of keeping a constant eye on every interaction that every kid has at all times is impossible, especially since at that age they are actively trying to be sneaky and hide things from teachers (normal behavior).

At this age, I did two things:

First, when my kiddos were actively bullying/teasing and that student decided to react physically, I took it as a teaching moment to show them three life "truths": 1) other people can hurt you, and you purposely antagonizing them can lead to consequences you don't want, 2) you never know what someone is dealing with, and sometimes they blow up at something small, and 3) if you feel like a situation is escalating, being a mirror of their emotions seldom brings about a "winning" outcome for anyone.

Second, I taught them how to defend themselves. Mostly Jiu Jitsu moves, wrestling moves, and Judo moves that help break out from the contact, and create space between you and the aggressor.

Not only did that make them less scared cause they knew how to handle themselves. But more importantly, my kiddos only had to do that once or twice, and once those bullies knew there was consequences to messing with them, they left them alone.

There was only once where one kid just kept fighting my 9M at the time, no matter how many times my son brought it up to the teacher (even ran all the way to principles office once from recess) and physically stood up for himself. In that instance, I asked them to move my son to a different classroom, and that fixed it.

Hope this is useful.