Speaking generally, I think we can just treat this in the standards of modern shounen anime?

Violence is A-Ok, and characters are allowed to explore their sexuality, but no on-screen sex, no portrayals of drug use by characters, and I feel like we can all agree that sexual harrassment and assault is super fucking awkward to just flatout write out in public? Generally I don't make a distinction in terms of the age of the characters myself; I just default to that sorta ruleset no matter how old the kiddos are.

I personally say 'one hit = loss' for the Duel setup though. Like, considering the presence of poisons and curses and all, just a scratch upon your Noble's pretty face could have massive consequences. Don't want the Knights to start approaching their actual bodyguarding careers with the reflex of "Nah, I can let two hits pass".

For TTRPGs, it largely just depends on whether or not you want to have stats to begin with. I'd say that flat modifiers and extra dice could exist together though. One indicates that a skill is more consistent, while the other one indicates that a skill is more risky. Could have people's natural dice pool exist in tiers too, so say, a beginner Knight naturally rolls with a 2d6, while a more experienced Knight naturally rolls with a 3d6.

The plot itself will definitely have to be figured out though. Right now, the state of the world itself seems kinda nebulous, so it's personally hard for me to imagine what would happen to the school. Like, what do Nobles do? Why are there Knights, rather than professional bodyguards with guns and shades? If there are Nobles, does that mean monarchies are still in place for powerful nations? What's America doing? etc.