I don't remember the last time a school assignment in anime was anything beyond a vehicle for a study session, but Rokudou gets great character mileage out of the diary as a representation of the fun being serious and studious in harmony with others can be... while getting his face smashed in. And all for Osanada's benefit. Like you said, the conviction is what swells the emotion in my chest as I watched. It feels like it is coming for somewhere sincere. The only thing going for Rokudou inherent to himself is his moral compass and drive to have (and help others have) a fun high school life.

If you look at similar gang-centric anime like Tokyo Revengers, the emotional stakes are much more pointed toward comradery and loyalty within the gang/friends without a broader view of a person's life and future implications. Holistically, getting Osanada's life back on track to integrate into society is much better for her in the long run, and that largely goes without saying to us who give it a little thought. Even her thug-buddy eventually relents and accepts his fate (more in line with gang thinking that self sacrifice is the greatest good in and of itself) when she tells him that not raising hell at school is preferable.

(Of course Colonel's diary was redacted like classified documents; the joke still hasn't gotten old)