Help with my thoughts about my kids

My kids are 6 and 4. My oldest has been getting into trouble at school for not being kind. I hear about or catch him kicking other kids sometimes. He calls kids names like butthead. He gets mad or what seems like careless and just does naughty things or mean things. He will trip a kid and laugh or ruin things around the house, by no accident.

I feel like I don’t know how to help him be his kind self. It seems like he’s looking for attention or he’s testing boundaries or something. I want him to be kind. That’s what we’ve taught him.
More than that I want to not be critical of him. I want him to feel loved and I want to show up for him in the best way I can to help him learn and grow. I feel like I do well with 3 or 4 instances every day that feel unkind, then by the time the 5 and 6 come around I’m just so disappointed and I am mad at him. I’m not helping him and I feel sick in this cycle where I have no control over teaching and loving the way I want to.
I want to be consistent, calm, loving, and peaceful in my parenting.