Hmm, that's a good question! I'm hoping others stop by (we've been really slow here, as you can probably see

) with some good ideas.
Some things that come to my mind from my previous schools (where I worked in mental health) include more administrative/student support level things like lunch groups to foster friendships and coping skills (e.g., problem-solving for friendships, manage anger or frustration, cope with divorce or similar circumstances, etc) and an open-door policy for a counselor or knowledgable office staff where students can go if needed. Of course, PBIS is supposed to be a good strategy as well, but I find it really varies in its implementation.
For classroom level things, the Positive Peer Reporting intervention is a good one
https://www.interventioncentral.org/...ive-behaviors-. One of my colleagues had a weekly problem-solving team meeting (whole class) where they worked out issues together. Sometimes it backfired a bit and created more drama, and sometimes it was great. That really relies on a skilled leader. At that school, we also had tickets we would give for good behavior and students would be randomly drawn. Very PBIS-like, but not a PBIS school.
That didn't really answer your question- in my previous schools, how necessary any of this was really depended on the administration and the individual teachers. We didn't have a school-wide system that was consistent by any stretch of the imagination. The school I most recently taught within also had a whole school morning meeting time and afternoon ending time (years later moved to just Monday morning and Friday afternoon because of how long it took). Our principal also did a lot of team-building activities with the staff. It was a pretty small school, but was a wide age range (K-12).
Interested in what others might have to say!