Does *peacefully* warning your classmate who is slightly being disruptive during lessons make you a mini-teacher? definitely not.
I wouldn't consider telling others that what they are doing is against the rules as backseat-moderation, I wouldn't ban someone for that. Threatening with bans (as mentioned in rule S9) is when you have crossed the blurry red line of backseat-moderation.

If I were a regular user and I really had to tell someone that what they did was bad (let's say that person was a close friend whom I cared about), I would do so in a PM *very peacefully* and not formally to avoid sounding like a mod; PMs are much more likely to be read than comments and it causes less embarrassment. If I didn't know that person well, I would just report the issue and entirely avoid saying anything in the comments. Doing so would save me from possibly being attacked by that person and possibly starting a flame war. I've come across so many what-you-are-doing-is-bad comments which caused flame wars and insults, so yeah, definitely avoid telling others in the comments publicly and just report silently.

This post might not be entirely related to the topic of this thread, but it's worth posting.

P.S. I might've angered lord jacob1 by posting here and not letting the thread die on its own.