My primary (private) school was the best that I've been to, in terms of discipline.
Discipline is instilled into everyone from the first term. The teachers are calm but firm, and they always speak, every time one of us messes up. Then, they escalate their voice if needed. Also, there was a Harry Potter style house points system which encouraged the majority of us to be well behaved as we're all in this together.
We had some people who always disrupted the class and they were allowed to come back, but every time they did something slightly wrong, they were sent outside for the rest of the lesson. I was quite well behaved for most of my school years and when I was sent outside, I was only there for a few minutes and then I came back. This way, teachers don't spend a lot of time shouting at my classmates, only maximum a few minutes per lesson.
The teacher made sure we queued for lunch in a single file. Single file were the two words that we heard a lot during those six years.
A combination of all of those made my primary school have very good discipline. I saw my state school teachers struggling to instill discipline but they didn't do most of the things my primary school teachers did.